Peking — What has changed in Sino-American relations as a result of President Reagan's visit to China? Mr. Reagan is back in the White House, preoccupied with Moscow, the Middle East, Central America, a recalcitrant Congress, and his own reelection campaign.
Here in Tian An Men Square, portraits of Marx, Engels, Lenin, and Stalin went up the day of the President's departure in preparation for May Day celebrations. The following evening, on May 1 itself, a magnificent fireworks display - the first on such a scale since China plunged into the chaos of the Cultural Revolution (1966-76) - reminded the world who invented gunpowder.
And now General Secretary Hu Yaobang of China's Communist Party has departed for a week of brotherhood and solidarity with North Korea and its longtime ruler , Marshal Kim Il Sung.
Having watched maneuverings involving the Korean Peninsula in Washington and Peking, Tokyo and Seoul, Marshal Kim may have decided to do a little card-playing himself. He will be going to Moscow in the latter half of this month.
Mid-May will see a Soviet delegation arriving in China, headed by First Deputy Premier Ivan Arkhipov, the highest-level Soviet official to visit China in many years. Mr. Arkhipov will ostensibly discuss economic relations with China - a topic he is well qualified for, since as Premier Zhao Ziyang told President Reagan, he was the top Soviet aid official in China during the Sino-Soviet honeymoon of the early 1950s.
Washington is taking the Arkhipov visit in stride. One of the results of Mr. Reagan's own visit is a more relaxed American attitude toward the prospect of better Sino-Soviet relations. Just as China has had to be satisfied with a Washington that shows no discernible haste to reduce its arms sales to Taiwan by the large amounts Peking would like, so Mr. Reagan has had to profess himself supportive of better Sino-Soviet relations as a factor for stability in the region.
The President has also said he respects China's insistence on remaining nonaligned and therefore does not seek an alliance with China. Yet Peking and Washington are discussing - albeit in a very preliminary stage - American arms sales to China. Defense Minister Zhang Aiping's visit to Washington in June will undoubtedly push these talks along.
The moves may seem contradictory, but taken together they reflect three major concerns of China today - concerns senior leader Deng Xiaoping and his associates Zhao and Hu took pains to get across to Reagan.
First, China's own policy for the forseeable future is irrevocably set on achieving modernization through an open-door policy of cooperation with the advanced industrialized countries.
That policy requires peace and stability around China, as well as within. Hence the move to defuse tensions with Moscow. Hence the effort to stabilize the Korean Peninsula. Hence the emphasis on peaceful reunification with Taiwan and assurances to Hong Kong of unchanged social and economic systems for up to 50 years after Britain surrenders control to China in 1997.
The United States, along with Japan, can be an important partner for China in this modernization effort. It can also play a role with regard to stabilizing Korea and Taiwan.
President Reagan shied away from any commitment regarding Taiwan, beyond what the US is already pledged to do in three joint communiques. But his visit symbolizes the stake that the US has in the success of China's modernization effort, an acknolwedgment that without such a success China itself cannot become a factor for peace and stability in the world.
Second, the Soviet Union remains the only real threat to China's security. Here too - despite President Reagan's acknowledgment that China will remain nonaligned - the United States can play a quiet role. It is only 360 miles from Peking to Soviet tanks on the Mongolian border, and the item the Chinese are most intersted in buying from the US is antitank weaponry.
Third, Taiwan remains the only major obstacle, ''the knot,'' as Mr. Deng put it, in Sino-American relations.
Reunification with Taiwan is an emotional issue here, which if mishandled can upset reason and logic.
This is an election year. in the US, and Mr. Reagan has many right-wing supporters. What Peking fears is that somehow a process may be started wherein American politicians vie with each other to show who is a better friend of Taiwan.
That could be disastrous to a Sino-American relationship which, for all its increasing warmth, is still in the stage of ''sprouting'' - a relationship to be nurtured with tender care.