WESTINGHOUSE and General Electric must be ecstatic. An early tangible accomplishment of President Reagan's visit to the People's Republic of China is the planned announcement Friday of conclusion of an agreement on commercial nuclear power cooperation. The agreement could mean billions of dollars' worth of business for Westinghouse and other American companies that can supply equipment and technology for China's ambitious nuclear energy program.
The agreement, which will be initialed April 30 before the President leaves China, underscores the importance that China and the United States place on their expanding trade and economic ties. Both nations are eager for business.
China looks to the US for help with its plans for modernization. The US sees in China an enormous market for American goods - from wheat to nuclear reactors.
''Technology has made us neighbors,'' the President said in a toast at a dinner hosted by Chinese President Li Xiannian soon after his arrival here.
''Neighbors are not family, but they can be dear and trusted friends. That is the spirit I sense already. It is something upon which weighty accomplishments can be built.''
Arriving in the Chinese capital Thursday for a six-day visit, President Reagan was given a warm, ceremonious welcome. For security reasons, Chinese citizens were not allowed to fill Tian An Men Square, which was bleak and empty under leaden skies. But Chinese military units marched crisply, the air rang with a 21-gun salute, and 200 schoolchildren were on hand, waving colorful paper flowers and shouting, ''Warmly welcome, warmly welcome.''
Reagan is the third American President to visit the People's Republic. Richard Nixon came here in 1972 and dramatically ended a generation of isolation between the two nations. Gerald Ford visited in 1975. Jimmy Carter, who established diplomatic relations with China in 1979, did not visit until after he had left office.
The Reagan journey has special interest - diplomatically and politically - because it represents a turnabout for the ideological President. This is the first time Reagan has set foot in a communist country as President, showing that he is prepared to deal with the communist world.
No American leader has denounced the ''evils'' of communism more vigorously or defended US ties with Taiwan more firmly. The Reagan visit thus points to a recognition that the US and China have common interests, despite their anti-thetical systems and differences of view.
''I have not come to China to hold forth on what divides us, but to build on what binds us,'' the President said in remarks prepared for delivery Friday to Chinese community leaders. ''I have not come to dwell on a closed-door past, but to urge that Americans and Chinese look to the future, because together we can and will make tomorrow a better day.''
What binds China and the US is more than technology. It is also a mutual interest in containing Soviet expansionism, especially in the Far East, and enhancing global and regional security. While President Reagan is expected to refrain from sharp attack on the Soviet Union during his visit here, he has not shied from explicit references to Soviet aggression.
''America and China both condemn military expansionism - the brutal occupation of Afghanistan, the crushing of Kampuchea - and we share a stake in preserving peace on the Korean Peninsula,'' he stated in his prepared remarks.
But in his first address to the Chinese people - there will also be an interview with Chinese journalists and a talk at Fudan University in Shanghai - the President dwelt largely on economic matters. Extolling China's efforts to loosen up its controlled economy, he stated that ''we are not surprised to see the fresh breezes of incentives and innovation sweeping positive changes across China.''
The text of his speech noted that US companies have invested almost $700 million in joint ventures and offshore oil exploration in China, making the US the largest foreign investor in the country. Chinese efforts to improve the climate for investment were hailed.
Conclusion of the agreement on nuclear cooperation ''will be based on shared principles of nonproliferation,'' the President said.
Negotiations for an agreement, begun in 1981, intensified in recent weeks. But it was only Thursday that the two sides resolved the contentious issues, US officials say. Among other things, the US was seeking assurances from Peking on US legal restrictions pertaining to control over the reprocessing of spent nuclear fuel and compliance with the Nuclear Nonproliferation Act.
China, in turn, sought language that would not appear to infringe on its sovereignty.
The proposed agreement, which must run through a legal process in the US before being signed, provides that (1) no nuclear materials or equipment will be used to build a nuclear explosive device, (2) no nuclear material or equipment will be transferred to a third country without the agreement of the US, and (3) no nuclear material will be enriched, reprocessed, or, in the case of weapons-usable materials, stored without prior US agreement.
The Chinese seem eager to make the presidential visit a success. Chinese news media have been commenting daily on the event.
On the evening of Reagan's arrival, they aired a program about his life, including scenes from his childhood. Also, the President's three speeches to the Chinese will be broadcast in full or in part over Chinese television, though not live.
''They made extremely generous provisions for the visit,'' said a senior administration official. ''The Chinese have really stretched themselves.''
Apart from everything else, the Chinese wanted President Reagan to visit because they felt he would be more pro-Taiwan if he did not. American experts suggest the Chinese leadership probably calculated Reagan would be in office for another four years.
Administration officials, in turn, see the visit as important for the President's education - to let him see Chinese leaders on their own turf and experience firsthand what China is trying to accomplish. ''This is important, especially given Reagan's earlier views,'' said an official, adding: ''He's come a long way in regard to his perspective on the China question.''
There will be considerable pomp and circumstance in the visit. The President has a heavy schedule of sightseeing, including the Great Wall and the ancient capital of Xian, events that will keep him on camera back home and enable him to dominate the news.
But beyond the media razzle-dazzle lies a significant foreign policy purpose - to stabilize and improve a relationship that has enormous strategic importance and economic potential for both countries.