President Reagan has decided to extend the life of the bipartisan Scowcroft commission for two years, administration officials said yesterday, in what one called "a guarantee of genuine follow-up on both arms control negotiations and the MX."

The Scowcroft panel, officially the President's Commission on Strategic Forces, has been the driving force behind the administration's thus-far successful effort to obtain congressional approval of the MX intercontinental ballistic missile after two earlier defeats.

The commission is scheduled to go out of existence on June 16, but officials said Reagan will extend its life to Jan. 1, 1985, with the expectation that it would become "institutionalized" and continue in some form either in his second term or in another presidency.

Continuation of the commission is likely to be hailed with approval by moderate congressional Democrats who voted for the MX after Reagan told them he is as committed to genuine arms control negotiations with the Soviets as he is to deploying 100 of the MX missiles in existing Minuteman silos.

Some moderates called for appointment of a continuing advisory unit on arms control as a condition of support for MX.

Administration sources said that Lt. Gen. Brent Scowcroft, a retired Air Force officer who served as national security affairs adviser to President Ford, had agreed to stay on as chairman, and that Thomas C. Reed, secretary of the Air Force in the Ford administration, would continue as vice chairman.

The retention of Reed, who has come under scrutiny by a federal grand jury and a congressional committee for a 1981 stock deal, could be controversial.

Reed resigned as White House consultant and deputy national security affairs adviser to the president after the Scowcroft commission submitted its report on April 6.

However, both the president and national security affairs adviser William P. Clark were said to be solidly in support of keeping Reed, the administration's MX expert, on the advisory panel, which would be headquartered in the Pentagon. Reed played a major role in Reagan's California gubernatorial campaigns, and served as first appointments secretary to Reagan as governor in 1967.

While Reagan won on MX last month, the White House recognizes that his support on the issue is potentially shaky and could disappear if the administration fails to adopt recommendations of the Scowcroft unit when the Strategic Arms Reduction Talks (START) resume with the Soviet Union in Geneva Wednesday.

The commission called for "vigorous pursuit of arms control," and recommended that the administration change its arms reduction proposal to count warheads instead of weapons.

Reagan has promised to do this and to propose some version of a "build-down" proposal advocated by Sens. Sam Nunn (D-Ga.) and William S. Cohen (R-Maine) in which both sides would remove more than one older nuclear weapon for each new one they add to their arsenals.

But exactly how to revise the START proposal has been a subject for debate in the administration.

White House spokesman Larry Speakes said the president is considering options this weekend at Camp David in preparation for a probable decision when the National Security Council meets Tuesday.

The Scowcroft commission also recommended development of a small, single-warhead missile ultimately designed to replace the MX and research and development on "hardening" of the Minuteman silos to give them better protection against a Soviet strike.

Administration officials said continuation of the commission would ensure follow-through on these objectives, and on research for an anti-ballistic missile defense.

Scowcroft agreed to continue as chairman after being assured that the workload would be distributed under a new committee system, officials said. They said the full commission probably would meet only two or three times annually.

The 11-man commission includes prominent national security and science officials in four administrations, including Reagan's former secretary of state, Alexander M. Haig Jr., former CIA director Richard M. Helms and President Carter's Pentagon chief of research and development, William J. Perry.

Among the counselors to the commission are Henry A. Kissinger and Carter's secretary of defense, Harold Brown, who is said to have played a major role in the commission's deliberations.

Former secretaries of defense Melvin R. Laird, Donald H. Rumsfeld and James R. Schlesinger Jr., Carter White House counselor Lloyd N. Cutler and former CIA director John McCone are the other counselors.