Former secretary of state Henry A. Kissinger said yesterday that he would have signed the SALT II treaty himself, thus endorsing its specific contents, but went on to warn the Senate that the United States has fallen into dire strategic straits.

Ratification of the arms limitation treaty, Kissinger said, should depend on three conditions - a new American military strategy and an expanded defense program to fulfill it, three understandings or reservations that the Senate could add to the treaty without requiring any renegotiation and a firm statement from the Senate linking future SALT negotiations to restrained Soviet behavior around the world.

The Soviet Union soon will have palpable strategic advantage over the United States, Kissinger said, and when other countries realize this, "we will face a crisis in our relations with the rest of the world."

Recent years have brought "revolutions" in the strategic balance - the end of clear-cut American superiority - and thus in "the postwar security and geopolitical structure," which he said was based on that American superiority.

"Rarely in history has a nation so passively accepted such a radical change in the military balance," Kissinger told the Foreign Relations Committee. Although he foresaw "very grave dangers" facing the United States in the 1980s, Kissinger said that by the late '80s the situation could become "really quite favorable" and that SALT II will permit the United States to take brisk steps to rectify its position.

In all, Kissinger spent nearly 5 1/i hours at the witness table in the grandiose Old Senate Caucus Room, first reading from the longest piece of prepared testimony yet presented in the SALT hearings, then answering questions. It was a vintage Kissinger performance, filled with intellectual agility and studied ambiguity.

Toward the end of the long hearing, Sen. Claiborne Pell (D-R.I.) complimented Kissinger. "You've got a marvelous diplomatic way of not being too precise sometimes8c Pell observed with a smile.

Kissing was precise on one point - the unilateral decisions of the Carter administration were largely to blame for the gravity of the situation he described. He said several times that he was not anxious "to assess blame," and he traced the underlying U.S. problem back a decade and a half. But when asked pointedly what had changed since the days when he was in power and giving rosier descriptions of the U.S. position, Kissinger listed mostly Carter administration decisions.

Specifically, he said, the Ford administration had been counting on deploying the B1 bomber (canceled by Carter) and on earlier availability of the MX missile (now not deployable before 1986) and the Trident submarine (coming into service next year, but at a slow pace).

Moreover, Kissinger said, the Soviet Union has been conducting a "geopolitical offensive" since 1975 that has transformed the nature of world politics to America's disadvantage. He scorned the notion that a "small Caribbean country" - Cuba - could send troops around the world almost at will while the United States could do nothing about it.

Perhaps reflecting the bizarre course of the SALT debate thus far, the Carter adminstration last night issued a statement welcoming Kissinger's general approach to the SALT II agreement and his opposition to its renegotiation or rejection." The statement said the administration does "not agree with some of the points made in Dr. Kissinger's analysis," but that his comments "contribute to a constructive debate on these vital foreign policy and defense issues."

The statement also revealed a little administration sensitivity: "The SALT debate has underscored the strategic problems which the president has recognized and has sought to rectify since he took office."

Senior administration officials said last night they took the Kissinger testimony as a plus in the SALT debate, though they acknowledge that meeting his conditions - like those of Sen. Sam Nunn (D-Ga.) and others who demand more defense spending in tandem with SALT II - will require political agility.

Kissinger said explicitly he agreed with Nunn. He said the Senate should delay action SALT II until it has received a new, supplemental defense appropriations bill and a revised five year defense program outlining an invigorated U.S. strategic policy.

In his prepared testimony Kissinger said the United States should accelerate development of the MX missile and Trident II submarine missile to give their silos; improve air defense abilities against the Soviet Backfire bomber; take "immediate steps" to restore the balance of medium-range nuclear weapons in Europe, and add to U.S. capacity to police farflung corners of the world, especially by improving the Navy.

Kissinger said the Senate shold demand an "obligatory commitment" from the administration for more arms spending.

Sen. Joseph R. Biden Jr. (D-Del.) told Kissinger he didn't understand "what you want." Biden listed new U.S. strategic and military programs already under way, and said, "We're moving rapidly to render the Soviet arsenal in the late 1980s much more vulnerable to a first strike than our forces would be."

The three treaty changes Kissinger proposed would declare that the accompanying protocol, which controls the use of cruise missiles that may be depolyed in Europe, not be extended beyond 1981, its expiration date; would declare that the Senatehs view that nothing in the "noncircumvention" clause of the treaty would interfere with normal U.S. aid to its allies; and would establish as a condition for a SALT II agreement the principle that the United States be entitled to match any Soviet weapons system.

Kissinger said this last idea is the best way to deal with the 308 "heavy" super missiles that SALT II allows the Soviets to keep but forbids the United States from matching.

None of thses changes would force a reopening of negotiations with the Soviet Union, Kissinger said, and administration officials later said he was probably right.

The one subject Kissinger raised yesterday that could upset the administration's SALT-selling program was linkage - the tying of arms control to general Soviet behavior around the world. This issue had all but died out of the SALT debate until yesterday, when Kissinger revived it energetically.

Kissinger said it was too late to tie ratification of SALT II to past Soviet behavior, but he urged the Senate to accompany ratification with a firm declaration that future negotiations would depend on the Soviets and their allies and proxies following restrained policies in the future by avoiding intervention in other nations. If the Soviets failed to live up to this declaration, Kissinger said, the Senate could vote to suspend whatever SALT negotiations were going on at the time.

Kissinger's remarks on linkage set him at odds with the Carter administration, which has concluded that SALT must stand on its own as a contribution to national security. Kissinger, however, argued that the United states has both the duty and the opportunity to impose "polticial restraint" on the Soviets as the price for cooperation in arms control.

Kissinger referred repeatedly to his fear that the shifting strategic balance will embolden the Soviets to take more risks in regional crises. At one point, he said he had failed to analyze fully the consequences of rough strategic equality on lower-level conflicts around the world. He acknowledged that the Nixon-Kissinger brand of detente had failed to testrain the Soviets.

Sen. Frank Church (D-Idaho), chairman of the Foreign Relations panel, challenged Kissinger, observing that overwhelming U.S. strategic superiority hadn't deterred the Soviets from building the Berlin Wall or turning Cuba into a military base. "I can't see how they'd be inhibited by our building the MX," Church said.

"But they might be less inhibited if we didn't," Kissinger replied.

Kissinger said the NATO allies are deeply ambivalent about SALT II - worried on one hand about the instability that would follow its rejection, but concerned also that the shifting strategic balance means "the basis of their security is eroding."

Two Republican senators, Jesse Helms (N.C.) and Howard H. Baker Jr. (Tenn.) sought to extract from Kissinger some encouragement for their views that the treaty might best be substantively amended before it is ratifiied. Kissinger gently but firmly rebuffled them.

On the issue of the Osvets' 308 missiles, which Baker has call a fatal flaw" in SALT II, Kissinger said there was no practical opportunity to do anything about them within the sic-year lifespan of SALT II, so the issue would be better dealt with in SALT III.

Despite this rebuff, Baker praised Kissinger's testimony highly, raising hopes among some SALT supporters that the minority leader might find a way to use Kissinger's testimony to move back toward approval of the treaty.

Kissinger told Helms that trying to renegotiate the treaty now would probably require a temporaty decision to recognize the SALT limits while talks went on, and this might prevent the United States from undertaking the buildup Kissinger said was necessary.

SALT, Kissinger said, can only provide confirmation of an existing power relationship. The Soviets will never make unilateral concessions, he said. "If we want equality, we must build to equality," he told the committee. CAPTION: Picture, HENRY A. KISSINGER . . . foresees "very grave dangers"