Peking — The prospect of warmer Sino-Soviet relations has sent ripples of uncertainty not only across Washington and other Western capitals but also through China's own Southeast Asian backyard.
To counter these ripples, China's paramount leader, Deng Xiaoping, has reassured Thai Prime Minister Prem Tinsulanond that the process of normalization with Moscow does not mean a diminution of China's resolve to oppose Soviet ''hegemonism'' or to get Vietnamese occupying troops out of Kampuchea.
As if to give point to this reassurance, China gave a full ceremonial welcome Nov. 21 to Son Sann, prime minister of the coalition recently formed to fight the Vietnamese-backed government of Kampuchea (Cambodia).
Both Huang Hua and Wu Xueqian, the old and new foreign ministers, attended the ceremony in Tiananmen Square presided over by Premier Zhao Ziyang.
Neither the change of foreign minister nor that of defense minister (both announced Nov. 19) is believed to mean a major shift in China's foreign or defense policies. Both changes had been expected for months and their timing is keyed to the approaching session of the National Peoples Congress, China's legislature, not to developments in Sino-Soviet relations.
Still, Bangkok and other Southeast Asian capitals are concerned about what changes in Sino-Soviet relations, if they take place, will mean for their own relations with Peking. Mr. Deng therefore used General Prem's visit as the occasion to state that normalization talks with Moscow ''are based on one principle - opposition to hegemonism and preservation of world peace.'' He added , ''We should firmly support the three patriotic forces of Kampuchea in their fight against the Vietnamese invaders.''
Since Chinese arms reach the Kampucheans through Thai territory, Thailand's continued support for the Kampuchean coalition is essential to the maintenance of armed resistance to the Vietnamese occupiers. The leadership here has gone to great lengths to preserve and strengthen its ties with Bangkok.
Thailand's political and military ties with Washington are also close. Thus, despite Mr. Deng's criticism of some aspects of American foreign policy, China and the United States are still pretty much in step with each other as regards Southeast Asia.
Meanwhile, the appointment of new foreign and defense ministers here is expected to bring more vigor to the execution of foreign and defense policy, rather than a realignment or a new direction in either field.
Mr. Wu, who is 61, is close to Communist Party General Secretary Hu Yaobang. He worked for many years in the Communist Youth League, which Mr. Hu headed. Mr. Wu has repeatedly led delegations overseas and is said to be fluent in English. One of his sons studied at an American university.
Zhang Aiping, 72, the new defense minister, hails from Sichuan, Mr. Deng's home province. A Long March veteran, General Zhang has headed the Defense Science Commission as well as being a deputy chief of the general staff.
Zhang, who replaces Geng Biao, is expected to bring new vigor to the drive to modernize the People's Liberation Army, which is 4.2 million men strong and the world's largest standing army. Modernization involves slimming down personnel, as well as upgrading of the quality of Chinese weapons and equipment, which some Western experts say are at least 20 years behind those of the world's major military powers.
Both reductions in personnel and upgrading weapons and technology mean a vastly changed People's Liberation Army. And Mr. Deng, who controls the armed forces as chairman of the Party's Military Commission, evidently intends General Zhang to spearhead the process.