President Reagan has received a letter from Soviet President Leonid I. Brezhnev setting the stage for the beginning of a new round of negotiations on reduction of strategic nuclear weapons, Secretary of State Alexander M. Haig Jr. said yesterday.

Haig, speaking on the television show "Face the Nation" (CBS, WDVM), said the next step would be to set a date for the U.S.-Soviet talks, possibly "through diplomatic channels at the State Department level." Later he told reporters that Soviet Ambassador to Washington Anatoliy F. Dobrynin may be bringing a date for the negotiations when he returns here from a visit to Moscow.

Another possibility, Haig suggested, is that final arrangements for the beginning of the negotiations could be worked out in the meeting expected to be held in New York next month between Haig and Soviet Foreign Minister Andrei A. Gromyko.

While declining to make public details of the Brezhnev letter, Haig characterized as "basically encouraging" the overall Soviet response to Reagan's May 9 call for resumption of strategic arms negotiations, this time with the objective of deep cuts in nuclear arsenals.

Official sources said the Brezhnev letter, which was received at the White House late last week, is parallel to the position that the Soviet leader took last Tuesday in his address to the Young Communist League in Moscow. In that address, the Soviet leader reaffirmed his nation's willingness to resume talks "without delay and without any strings attached" and declared that Moscow has always favored substantial reductions.

Haig, commenting on the Brezhnev speech, said "one can only be encouraged" by those two points.

On the other hand, the Soviet leader in his address criticized Reagan's arms proposals as "absolutely unilateral in nature . . . unrealistic . . . perhaps simply an insincere position" that prejudices Soviet security while leaving Washington free to continue stockpiling nuclear arms.

Haig may have had these statements in mind when he said yesterday that Brezhnev's address also contained "a number of self-serving posturing statements of propagandistic character."

The secretary of state, in yesterday's interview program, argued anew against the two broad alternatives that have been proposed to the Reagan strategic arms position:

A freeze on the large-scale nuclear buildups that are under way, as proposed by Brezhnev and by anti-nuclear weapons groups in the West, "would lock the United States into positions of inferiority in key areas," according to Haig. He mentioned particularly Western Europe, where the United States is proposing to place medium-range atomic weapons to counter similar weapons on Soviet soil.

Ratification of the SALT II treaty as a basis for further negotiations and reductions would "lock in" flaws in that document, Haig argued. He mentioned an advantage for the Russians in heavy missiles, unspecified "deficiencies" in verification and the absense of formal restraints in the treaty text on the Soviet Backfire bomber.

At the same time Haig conceded "there were many good aspects" of SALT II and said the two superpowers continue to observe some of the restraints, in their mutual interest, "because it provides an international backdrop of greater confidence on which to proceed" into the new negotiations.

Discussing the same point, former secretary of state Edmund S. Muskie proposed yesterday that Congress pass a formal declaration that the United States will continue to abide by terms of SALT II as long as the Soviets do likewise.

Muskie, in a commencement address at Marquette University, argued that "it is only sensible not to throw out the baby with the bathwater" by abandoning all the limitations agreed upon until now. Noting that the SALT II "statement of principles" signed by the United States and the Soviet Union in 1978 calls for additional talks on "significant and substantial reductions," Muskie proposed that negotiators start from there and aim for gradual annual reductions from SALT II levels with a goal of an overall cut of 50 percent in limits and sublimits over five to 10 years.

Such a result would bring the final nuclear arms totals "into the ballpark of the Reagan plan," Muskie said.