President Reagan yesterday fired the director of the U.S. Arms Control and Disarmament Agency, Eugene V. Rostow, 69, a conservative Democrat who had come under fire from some conservative Republicans for his choice of aides and from the White House for how he managed his agency.

In a major shake-up of several top ACDA officials, Reagan said he was nominating Kenneth L. Adelman, 36, deputy U.S. representative to the United Nations, to replace Rostow, and David F. Emery, a former Republican U.S. representative from Maine, as Adelman's deputy.

The president also announced that he was accepting the resignation of Richard Staar as the chief U.S. negotiator in the long-running Mutual and Balanced Force Reduction talks in Vienna, meant to seek cuts in conventional forces in Europe such as tanks, artillery and non-nuclear missiles.

Staar had also run into problems over his job performance, and his removal had been sought by the State Department, in part because of erratic behavior and an obsession with security, according to administration officials.

Reagan yesterday nominated a career Foreign Service officer and former ambassador, Morton Abramowitz, to replace Staar.

Although the president said he was accepting Rostow's resignation with regret and noted that Rostow had served "this and other administrations with distinction," a brief statement by Rostow made clear that he had been fired.

"It has been a privilege to serve as director," Rostow said through a spokesman. But, he added, "in recent days it has become clear that the president wished to make changes. In response to his request, I have tendered my resignation."

Rostow, a former Yale law school dean and professor who supported Reagan for the presidency and is a leading advocate of a strong military, was brought to Washington 20 months ago to head the agency that is supposed to help develop policy on such crucial issues as nuclear arms control negotiations with the Soviet Union. But he has had a stormy tenure.

White House deputy press secretary Larry Speakes, in describing the actions, said, "It was a consensus of the president and his foreign policy advisers that it would be helpful to make some changes in the management of his arms-control negotiating team."

Speakes said Reagan "believes these changes will enable the process to move forward, unimpaired, toward that goal we all seek, true arms reduction. The new members of his team will provide aggressive leadership in this important area of American policy."

Other officials said privately that the removal of Rostow was due in part to a decision by Reagan to personally "take charge" of the administration's controversial arms-control policies, to involve Secretary of State George P. Shultz more intensely in that policy, and to put together what officials called "a more cohesive team."

The idea, officials said, was to do this during the current recess in Geneva of the two major negotiations with Moscow on reducing strategic and intermediate-range nuclear missiles.

Rostow also has had health problems, including a recent hip operation that kept him out of Washington for several weeks, and White House aides said Reagan "needed somebody who could give full-time management attention."

Rostow is known to have irritated some White House officials by providing unsolicited advice on issues outside of his responsibilities, such as on Middle East policy. But the crucial issue that ultimately made his situation untenable was the victory last week of a small group of conservatives, led by Sen. Jesse Helms (R-N.C.), who forced the ouster of Rostow's choice as his key deputy, Robert Grey.

The White House, which once had promised to support Grey, decided against it.

In the aftermath of Grey's ouster, Rostow told reporters that he thought his opponents were opposed to arms-control agreements and were trying to take over U.S. arms-control policy.

He said such disproportionate influence by this small group in Congress could promote a "fear response" by the Soviets and that he, Rostow, was trying to steer a course between "people who want an agreement with the Russians at any price . . . and people opposed to having any agreement at all."

The irony of Reagan's nomination of Abramowitz, who is highly regarded among career diplomats, is that he was one of the first victims of the conservatives in Congress, who helped thwart his nomination as ambassador to Indonesia.

While Rostow had problems in some places, other officials in the State Department point out that his intellect and articulateness were widely appreciated among West European allies, and that his removal at this time could cause additional uncertainty in Europe about U.S. arms-control policies.

His firing comes at a critical moment in the development of Reagan's arms-control policies.

For example, senior officials said yesterday that, despite some pressures for change within the administration, the United States will stick with its original proposals for nuclear arms reductions when negotiations with Moscow resume in Geneva in three weeks on both the START talks on strategic missiles and bombers and the INF talks on intermediate-range missiles based in Europe.

That approach is known to have been questioned by U.S. negotiator Paul H. Nitze, 76, who heads the U.S. delegation at the INF talks, which are meant to reduce the European-based missile forces.

Nitze is understood to want authority to explore compromise solutions other than Reagan's original "zero-option" proposal, which calls for dismantlement of about 600 existing Soviet missiles in return for the United States not deploying 572 missiles starting in December.

Nitze, sources say, believes such authority is needed to find out whether an eventual agreement with Moscow is possible and to combat recent public statements by the new Soviet leader, Yuri V. Andropov, that are meant to portray Moscow as more flexible than Washington on this crucial issue.

Nitze is also said to favor some public statement by the administration that the United States, while still believing that the zero-option is the best solution, stands ready to at least explore other alternatives based on equal numbers of missiles, hopefully at far lower levels than Moscow is proposing.

In this view, such a statement prior to the reopening of the INF talks on Jan. 27 would help counter Andropov's public diplomacy and could ease the pressure on allied governments in western Europe that are pledged to support Reagan's plan but face potentially stiff public opposition. This could be especially important in West Germany, where crucial elections are scheduled for March 6.

But the current assessment of the White House is that such explorations would be premature until the Soviets come forward with much better proposals than they have thus far.

Yesterday, Reagan seemed to make this clear by repeating once again his basic proposals in both the START and INF talks. Privately, White House officials emphasized that "we are very much locked onto the zero-option. It is very clear that is the president's position. Nitze's instructions will be, 'Don't explore, listen for new Soviet proposals.' "

Nitze, who last summer undertook some independent explorations with his Soviet counterpart in Geneva, is to carry his case directly to Reagan within the next week, an aide said. Final instructions to the U.S. negotiators and an administration assessment of the overall arms-control situation are expected at a National Security Council meeting in the last week of January.

In 1976, Adelman served as a special assistant to Secretary of Defense Donald Rumsfeld. More recently, he was active in leading U.S. opposition to the U.N. Law of the Sea Treaty as he traveled with Rumsfeld, a special presidential representative on the issue, to leading Western nations in an effort to enlist support against the treaty.

Adelman is on close terms with many of the administration's more ideologically conservative supporters and has been pushed by some of them as someone who should have greater responsibilities in the foreign policy area.

Although his service to Reagan has largely been divorced from arms-control matters, he is regarded as generally knowledgeable about the issues, mostly on the basis of his earlier Pentagon experience.