President Reagan is prepared to lift the grain embargo against the Soviet Union in the next few weeks if he becomes convinced that the Russians do not intend to invade Poland, well-placed administration sources said yesterday.

These sources emphasized that no discussions are contemplated with the Soviets on the issue, and that the action, if it comes, will be based on U.S. evaluation of their intentions.

"In the last analysis, what we do will be based on deduction," one official said.

During the 1980 presidential campaign, Reagan repeatedly criticized President Carter for imposing the embargo and promised to lift it if he were elected. In a March 27 interview with The Washington Post in the Oval Office the president said again that he wanted to lift the embargo, but added: "I do not see how we could lift it at this time without sending the wrong signal."

The Soviet threat to Poland has been the barrier to lifting the embargo since Reagan took office. The issue came up in the first Cabinet meeting, and Secretary of State Alexander M. Haig Jr. raised the Polish question in a manner that White House officials said was persuasive to the president at the time.

Ever since, Reagan has rarely missed an opportunity when the issue has come up in private to make the point that he opposes the embargo on grounds that it is selectively unfair to farmers. This position has been reinforced by "a sense of the Senate" resolution calling for the lifting of the embargo and by meetings with farm group constituents, who are generally pro-Reagan except for the embargo issue.

But the president's promise and his basic political position on this issue ran headlong into Reagan's own determination to project a tough stance against the Russians. Within days of taking office, it became evident that this stance would have little meaning if the United States gave the Soviets a vital concession at the same time Russian troops were ringing Poland and threatening invasion.

There is a danger to the administration, however, if the decision to sell grain to the Russians is based solely on the situation in Poland.

By considering the future of the grain embargo in the context of the volatile Polish situation, the administration faces a serious problem of when to make a decision about the likelihood of Soviet intervention and what evidence or estimates to use.

Informed government sources said that at an early stage of the policy debate, the high councils of the administration considered setting either May 1 or June 1 as the date for a decision, making it known that the embargo would be lifted if the Polish situation were quiet at the time.

The sources said this course was rejected because of the realization that the threat of Soviet intervention is unlikely to disappear quickly and that circumstances on any given date could be so murky and ambiguous that a decision would be difficult. Even if the Russians do not intervene, U.S. intelligence estimates are that the situation in and around Poland is likely to remain tense for some time.

One date now in mind is April 27, the deadline for preparing the 1981 farm bill and the day Congress returns from its Easter recess. But this does not necessarily mean the embargo would be lifted then. One source yesterday cautioned against "picking a date certain" to end the embargo, and another said that the administration instead might inform Congress of its intention without setting any date for actually ending the embargo.

On Wednesday, Sen. Roger Jepsen (R-Iowa) said he had been assured by the White House that if the Polish situation remains stable the embargo will be lifted "within three weeks."

Some decision on whether to sell grain to the Russians must be made before Sept. 30, when a five-year agreement in which the U.S. agreed to sell at least 6 million metric tons of grain annually to the Soviets expires.

A Reuter dispatch from Moscow yesterday quoted a touring congressman, Rep. F. James Sensenbrenner Jr. (R-Wis.) as saying that Russian foreign ministry officials had told him that the Soviets would be interested in a new five-year agreement only if the United States made financial guarantees that it would not resort to a political embargo.