The Reagan administration may be ready to begin strategic arms negotiations with the Soviet Union as early as February, Secretary of State Alexander M. Haig Jr. and arms control administrator Eugene Rostow told a Senate committee yesterday.

The February date is the earliest publicly presented by administration officials. It was described by Haig and Rostow, in testimony before the Senate Foreign Relations Committee, as an expectation if all goes well rather than as a target date or promise.

Haig, who plans to meet Soviet Foreign Minister Andrei F. Gromyko in January in Geneva for another round of broad Soviet-American talks, said that circumstances suggest "February, March or April" for the beginning of new strategic arms negotiations if President Reagan can make decisions on U.S. proposals without delay and no major change occurs in the international environment.

While new negotiations would continue the process that produced the 1979 Strategic Arms Limitation Treaty (SALT II), the U.S.-proposed changes from that unratified document are likely to be "very substantial," according to Rostow. As many as six different approaches to the negotiations are under consideration within the administration, he said.

Haig, in his prepared testimony, sought to buttress the case for Reagan's controversial B1 bomber and MX missile proposals by tying them to progress in strategic arms negotiations.

More than any other elements of the U.S. weapons buildup, Haig said, "B1 and MX, and the degree of Congress' support for them, will make or break our attempt to negotiate a reasonable arms-control agreement."

The senators did not question Haig in detail on this statement. Nor were there questions arising from lengthy and recent statements on strategic and Euromissile arms-control issues by Soviet President Leonid I. Brezhnev.

Regarding the forthcoming negotiations with the Soviet Union on limitation of theater nuclear weapons in Europe, Haig described the U.S. objective as "a verifiable agreement that would achieve significant reductions on both sides, leading to equal ceilings at the lowest possible levels, levels which, ideally, could be zero."

The so-called "zero option" of eliminating all weapons covered by a Euromissile agreement is considered an unrealistic negotiating goal by many American officials, but some of them recognize the public relations asset in Europe of such an aim. None of the senators asked Haig to amplify his statement on the "zero option."

Asked about Reagan's recent controversial statement suggesting that a nuclear war might be limited to Europe, Haig called it "precisely right, and in keeping with NATO strategy and doctrine."

A former NATO military commander, Haig said the alliance seeks to limit hostilities to the lowest level possible, nuclear or conventional. He reported that NATO has contingency plans to fire a nuclear weapon for demonstration purposes under some circumstances. He gave no details, nor was he asked for any.

In a related development, the chief of the Strategic Air Command, Gen. B.F. Davis, testified yesterday before the strategic and theater nuclear forces subcommittee of the Senate Armed Services Committee.

In his prepared statement, Davis praised Reagan's commitment to strengthen U.S. strategic forces as a significant beginning in halting the slide in defense.

But Davis also made clear that the "the Air Force and SAC would have preferred to deploy" the new MX missile in the multiple-shelter scheme advocated by the Carter administration rather than in the existing Titan and Minuteman silos, as decided by Reagan.

Davis was questioned intensively in open session by Sen. Sam Nunn (D-Ga.) about whether the Reagan program in effect had extended the so-called "window of vulnerability" period in which the U.S. forces are vulnerable to attack.

According to accounts provided by several congressional aides, Davis answered by saying that he also would have preferred retention of the Titan missiles and B52 bombers being retired under Reagan's strategic weapons program, and acknowledged that U.S. strategic capability will be decreasing rather than increasing in the next four or five years.

Nunn also reportedly extracted from Davis an acknowledgment that modifying existing Minuteman silos to allow housing of the larger MX and make them strong enough to withstand a Soviet missile attack probably would involve technical violations of past SALT agreements restricting increase in volume in existing silos.

Although the first SALT agreement has expired and the second was never ratifed, the United States and the Soviet Union adhere to the provisions.