The crucial factor in Sino-Soviet relations is that China still perceives the Soviet Union as a direct military threat to its security. A second round of Sino-Soviet talks in Moscow has failed to ease this perception.
As the second round of talks ended last week, Foreign Ministry spokesman Qi Huaiyuan called the discussions ''useful'' and characterized the atmosphere as ''frank and calm.'' But a Chinese official told a visiting Japanese Socialist Party delegation that China had turned down a Soviet proposal for a joint document pledging mutual nonaggres-sion and security on their long border.
Such a document would be pointless, the official told the Japanese, because it did not include Mongolia, where the Soviets have several divisions, and because it did not come to grips with the three ''obstacles'' to improved relations that the Chinese have consistently emphasized.
The three obstacles are: large numbers of Soviet troops (as many as 1 million , Chinese leader Deng Xiaoping has said) on the Sino-Soviet and Sino-Mongolian borders, the Soviet invasion of Afghanistan, and Soviet support for the Vietnamese occupation of Kampuchea.
All three obstacles concern Soviet actions in areas surrounding China. Taken together, they are an attempt to encircle China, Peking officials say. Deputy Foreign Minister Qian Qichen, China's special envoy, returned to Peking from Moscow March 22 after five meetings with his Soviet counterpart, Leonid Ilyichov , and a final meeting with Foreign Minister Andrei Gromyko. Mr. Qian confirmed that a third round of talks would be held in Peking at an indeterminate date.
''China's principled stand on the question of eliminating obstacles in Sino-Soviet relations is firm and well-known to all,'' spokesman Qi said March 22. It would appear that Moscow had proposed a gradualist approach in a series of confidence-building measures and that China had rejected this approach, holding out for more specific action by the Soviet Union on at least one of the three major obstacles.
This Chinese attitude has been consistent since the first round of talks held in Peking last October between Messrs. Qian and Ilyichov. This does not mean that there has been no movement at all in Sino-Soviet relations. China has been signing annual trade agreements with the Soviet Union for the past several years. This year's agreement, signed in Moscow March 10, could mean a jump in Sino-Soviet trade from about $300 million last year to $800 million this year, according to Chinese sources.
But on military matters mutual suspicions remain high. The Soviets say of China's three ''obstacles'' that they concern third countries and that China should talk directly with Mongolia or with Vietnam to find out why they fear China and seek Soviet assistance.
China's response has been sharp. Whether the question was Afghanistan or Mongolia or Vietnam or Kampuchea, a New China News Agency commentary said March 10, these problems said to involve third countries ''all stem either from the use of military force by the Soviet Union or by other countries with Soviet support, or from Soviet deployment of armed forces in other countries. They constitute a grave threat to the security of China in particular and the peace and stability of Asia and the world as a whole.''