President Reagan has decided to cancel further production of one of the major new weapons in the Air Force arsenal, the air-launched cruise missile, which President Carter said was so good that the nation could safely do without the penetrating B1 bomber.

Because the Air Force says it fears that Soviet air defenses are improving and may soon be able to shoot down today's cruise missiles, Reagan instead intends to gamble on tomorrow's "Stealth" technology and build a brand new weapon.

The Air Force is already at work on a Stealth bomber. The Stealth technology involves a combination of techniques to evade enemy radar, including shaping planes or missiles so that radar beams glide over them.

Reagan's decision, unless reversed by Congress, will touch off a scramble among three of the nation's biggest defense contractors and raise fresh questions about the ability of bombers and subsonic missiles like the cruise to penetrate upgraded Soviet defenses.

Reagan's change of course on the air-launched cruise missiles does not affect the administration's plan to base other versions of the weapon on the ground in Europe late this year. The idea is to build Stealth advances into the strategic air-launched cruise missiles.

Reagan would stop the air-launched cruise program about a third of the way to completion, purchasing 1,499 of the 4,348 missiles the Air Force had intended to buy. The president's decision will cut the program's projected cost from $8.4 billion to $4.3 billion, the Air Force said yesterday. But the difference, and then some, will be spent on the new weapon, called the advanced cruise missile.

Air Force Secretary Verne Orr and Gen. Charles A. Gabriel, Air Force chief of staff, talked only sparingly yesterday of what they perceive as the greatly increased threat to the one-time wonder weapon, the ALCM B, the air-launched cruise missile recently installed on a squadron of B52s.

"Soviet air defense improvements are extensive," they said in their annual posture statement submitted to Congress. They said the Soviet SA10 antiaircraft missile has "some capability against cruise missiles." They added that Soviet aircraft--fighters and AWACS (airborne warning and control system)--are getting radars that can look down and pick out invading planes from the ground clutter shown on radar scopes.

The ALCM B is like a small drone aircraft. It flies slowly, but low, after being launched. The idea is to hide the missile in the ground clutter, which usually confuses ground-based radar warning stations.

While flying along, the missile's radar "feels" mountains and other features on the ground below and compares them with the map memorized by its mechanical brain. The missile corrects its course to keep on the path that has been plotted on its map. The ALCM B is credited with the ability to deliver a nuclear or conventional warhead with pinpoint accuracy on a target up to 1,500 miles away.

However, the Soviets' growing ability to put antiaircraft radars aloft in planes where they can "see" better, together with the Air Force desire for a longer-reaching ALCM so bombers can launch from a safer distance, has provided impetus for switching from the ALCM B to Stealth cruise missile.

Reagan's decision, unless reversed by Congress, means that the Boeing Co. of Seattle, prime contractor on the $8.4 billion ALCM B program, will have to scramble for new business to keep its new $27 million cruise missile plant in Kent, Wash., operating. The state's congressional delegation, which includes Sen. Henry M. Jackson (D-Wash.), ranking Democrat on the Senate Armed Services Committee, is expected to be active in this regard.

Boeing, General Dynamics Corp. and the Lockheed Corp. are the prime contenders for the new advanced cruise missile contract to be awarded next month. This time last year Reagan told Congress he intended to spend $872 million to buy 440 ALCM B cruise missiles from Boeing alone in fiscal 1984.

But there are no orders for the ALCM B in his fiscal 1984 budget just sent to Congress. A Boeing official said previous Air Force orders will keep the Kent plant in full production until 1986.

Carter on June 30, 1977, announced that he had decided against putting the B1 bomber into production. He said the B1 was "a very expensive weapons system, basically conceived in the absence of the cruise missile factor." Reagan, shortly after taking office in 1981, ordered the B1 into production and continued the air-launched cruise missile program as well.

The Air Force armed its first squadron of B52 bombers with cruise missiles in December. Reagan's plan to switch to a more advanced missile suggests that the Soviet air defense has improved faster than anticipated by the U.S. intelligence community.