The multi-billion-dollar wheat and corn deal signed by the United States and China today provides for the annual purchase of at least 6 million metric tons of American grain from 1981 through 1984.

The agreement, negotiated here over the last month, could improve President Carter's electoral prospects in the farm belt.

[In Washington, Secretary of Agriculture Bob Bergland said the accord provides for a "stable, high level" of trade and he stressed that the Chinese can be expected to buy more than the 9 million tons of grain in some years, although that is the maximum covered in the agreement.]

China is already expected to spend an estimated $2 billion for American farm products this fiscal year, including 6 million tons of wheat, 2.5 million tons of corn, nearly 1 million tons of soybeans and 2 million bales of cotton.

The agreement is likely to boost wheat prices and help make up for losses by American farmers because of the Carter administration's embargo on most grain shipments to the Soviet Union. Politicians in the Midwest, hearing of the deal earlier through news leaks, have disagreed over whether the publicity generated by the deal would be enough to win over farmers disenchanted with Carter.

For Peking, the agreement guarantees grain shipments at a time when it is straining to increase living standards. Signed by U.S. Ambassador Leonard Woodcock and Chinese Foreign Trade Minister Li Qiang, it also signals the good health of Sino-American ties despite recent Chinese criticism of Washington's dealings with Taiwan.

Diplomats here have emphasized the importance of the pact in the growing ties between the world's most populous and the wealthiest nations, and in smoothing out the U.S.-Chinese grain trade. Since 1972, when trade between the two countries resumed, grain sales to China have in some years been zero, and in other years as much as 4 million tons.

China purchased on an average about 1.1 million tons of wheat and 800,000 tons of corn each year from 1973 to 1979. Its total annual grain imports during that period were about 5.5 million tons.

China promises under the agreement to warn the United States if it plans to purchase more than 9 million tons of grain in a year or cannot purchase the agreed upon 6 million tons because of "exceptional circumstances" affecting all its foreign purchases.

The United States promises to warn the Chinese if it cannot sell the full 6 million tons annually as agreed, or if it cannot sell more than 9 million tons in years when the Chinese ask for that much.

Carter administration officials had earlier expressed some doubts about a long-term grain pact with China, for fear that a bad harvest would make it difficult to gather the 6 million tons without major price hikes.

Today's agreement provides that 15 to 20 percent of each year's grain sale will be in corn.