China is sending a high-level delegation to the Soviet Union next month to sign an agreement which is expected to boost trade between the two nations substantially, according to diplomats here.

The visit to Moscow by a delegation headed by China's Vice Premier Yao Yilin is also expected to further improve other aspects of the relationship between the two Communist rivals.

Aside from the visit by Chinese Vice Premier Li Peng to the funeral of the late president Konstantin Chernenko in mid-March, Vice Premier Yao's mission to Moscow will be the highest-level visit of its kind in more than 15 years.

Yao will visit the Soviet Union in early July for discussions with Soviet leaders on Sino-Soviet economic and trade developments and sign a five-year economic and trade agreement, a Chinese Foreign Ministry spokesman announced today. No details were released, but Asian and Western diplomats have said the agreement could boost trade to as high as $5 billion to $6 billion a year by 1991, or about the current level of U.S.-China trade.

The pact, agreed to during a visit to Peking last December by Soviet First Deputy Premier Ivan Arkhipov, covers the period from 1986 to 1990. Yao's visit returns the trip by Arkhipov, the most senior Soviet leader to come here in 15 years.

A gradual improvement in relations and an increase in day-to-day exchanges between China and the Soviet Union has been evident in recent months. Tensions have further subsided on the Sino-Soviet border and border trade is increasing. Consulates also are being reopened in Shanghai and Leningrad.

Following the exchange of greetings between the Communist parties of China and the Soviet Union earlier this year, the first in more than two decades, the Chinese made several other symbolic gestures apparently aimed at improving the atmosphere for normalizing relations.

But in April, while the sixth round of normalization talks were going on in Moscow, Deng Xiaoping, China's principal leader, reiterated that the Soviet Union could improve relations by gradually removing three obstacles -- inluding the Soviet forces along the Chinese border, Soviet support for Vietnam's occupation of Cambodia, and the Soviet invasion of Afghanistan.

"The Soviets took Deng's remarks badly," said one diplomat here. "In their view, once the number one man had spoken, the other signals from the Chinese didn't count."

The limits on improved relations were further stressed more recently when Huan Xiang, an adviser on foreign affairs to China's State Council, told reporters in a June 21 interview that despite his opinion that relations should be further improved, "in the foreseeable future or even longer, Sino-Soviet relations will stand still."

The Chinese appear to have reached a consensus that the Soviet Union now poses much less a threat to China than it did just a few years ago. The official Peking Review, in its June 17 issue, carried an article which argued that in economic and military affairs, the United States seemed now to "eke out an edge over the Soviet Union in their rivalry for global supremacy."

An Asian diplomat added that "saying that the Soviets are on the defensive and the Americans on the offensive implies a readjustment."