Secretary of State Edmund S. Muskie, calling SALT II an American "secret weapon," campaigned yesterday for improving U.S. national security through arms control as well as buildups.

Muskie, speaking to the Pittsburgh World Affairs Council, seemed to be putting into practice his recently expressed view that the strategic arms limitation treaty should be debated this fall to create a mandate for its ratification should President Carter win the Nov. 4 election.

Muskie did not mention in his supposedly nonpolitical speech that the Republican presidential candidate, Ronald Reagan, as well as the GOP platform, is opposed to the treaty with the Soviet Union that was signed in June 1979 The secretary of state has made this point on previous occasions.

In an address which concentrated on military issues, Muskie, who inadvertently referred to himself as "secretary of defense" in several recent appearances, compared SALT to a secret weapons system.

"By itself, this secret weapon would knock out about a fourth of all longrange Soviet Missiles and bombers that we project for 1985. It would do that without launching a nuclear war;

"The secret weapon has surveillance capabilities. With it, we will be able to keep better track of Soviet forces and programs," Muskie said.

The secretary of state also said that the "secretary weapon":

Would be compatible with other U.S. strategic programs such as the MX missile, Trident submarine and air-launched cruise missiles.

Would not require massive new funds, and would save money in the long run.

Does not worry U.S. allies, who are only concerned that the United States might not adopt it.

Without naming any of its opponents, Muskie declared "it may well be that some of those who oppose SALT I would support it -- even insist on it -- if it were a defense expenditure that could buy the same results."

Muskie said early this week that it is unlikely the Senate could consider approval of SALT II in a post-election session this year even if Carter wins. But he and others in the administration have said that to avoid the necessity for renegotiating some parts, a vote should be held by early next year.