Senior Reagan administration advisers yesterday tentatively decided that the United States should extend for one year the existing grain agreement with the Soviet Union, an administration source indicated last night.

The one-year extension emerged as the least objectionable of the options available to President Reagan, who has ruled out an embargo but doesn't want to renegotiate a new long-term agreement because of continued martial law in Poland.

CBS News reported last night that Reagan had reached a final decision to extend grain sales at their current level. However, White House spokesman Mort Allin said, "The president has not made a final decision on the matter."

But administration sources confirmed that Reagan's top economic and national security advisers met to discuss the grain agreement, which originally was a five-year pact, and, as one source said, came "close to consensus on a recommendation."

A set of options on the grain pact, which expires in September, was presented at a July 15 Cabinet meeting. Subsequent to the Cabinet meeting, grain sales were discussed by the National Security Council and then by the senior economic and national security advisers.

CBS said the grain agreement will be renegotiated with minimum purchase levels unchanged.

The current agreement requires the Kremlin to buy 6 million metric tons of U.S. wheat and corn a year and allows as much as 8 million tons. Last October, the U.S. agreed to allow purchases to reach 23 million tons this year, and the Soviets so far have bought almost 14 million tons.

American farmers have pressed for continuation of sales at higher levels because of a domestic grain surplus and depressed farm income.