WHAT CAN the secretary of state possibly mean when he declares that the SALT II treaty is "dead"? Though the treaty, negotiated by the Nixon, Ford and Carter administrations, has not been ratified, most of its terms are being respected by the United States and the Soviet Union. Both countries think it is in their interest. And so it is. Far from being dead, the treaty is quietly alive and serving the extraordinarily useful purpose of containing certain aspects of Soviet-American strategic competition while the two countries figure out what to do next. Indeed, it is precisely because the terms are alive that Mr. Reagan can continue to indulge the fantasy that the treaty is dead.

Why is the administration so churlish about SALT II? Apparently, the president cannot bring himself to embrace the handiwork of his predecessors, especially Jimmy Carter. In no small measure, he rode to the White House by campaigning against SALT II and all the lapses of strategic necessity and national will it was held to embody. Is this not the moment, however, for Mr. Reagan to reconsider? It is not simply that he is now the president and that, approaching a new negotiation, it is his turn to call for a bipartisan rallying around. The principal reason why he and others spurned SALT II--to alert the nation to a fresh sense of its needs in strategy and leadership--dissolved when he assumed office and undertook his new program.

The administration contends that the treaty is "fatally flawed": it "leaves" Moscow the advantage of its big land-based missiles. But it is not the treaty that leaves Moscow with that throw-weight advantage; it is the years-in-the-building structure of the two countries' forces. In fact, the treaty is working to restrain that advantage: the warhead limit on big missiles is being informally observed. If the treaty had been ratified, it would actually have diminished that advantage: Moscow would have had to dismantle 10 percent of its most threatening weapons.

Perhaps it made sense to Mr. Reagan, while he was playing hard-to-get on arms control, to decry SALT II. But he is heading for START. There is no inconsistency between SALT and START--START would simply go further. Some of the formulas worked out in SALT are available to be applied or recycled in START. It could only make the Soviets take Mr. Reagan's proposals more seriously if he acknowledged the element of continuity in American policy. At home, meanwhile, a more forthcoming attitude toward past arms control efforts would surely help Mr. Reagan gather the support he will need for his own.