A senior administration official asserted yesterday that arms control planning within the Reagan administration is now "going very well" and that the United States may be ready to renew negotiations with the Soviets on reducing long-range strategic nuclear weapons earlier than previous public estimates.

In June, Eugene V. Rostow, then the director-designate for the U.S. Arms Control and Disarmament Agency, told Congress it would probably be March, 1982, before the new administration would be in a position to enter negotiations with Moscow on further limits of ocean-spanning missiles and bombers. Yesterday, the official who spoke with reporters at The Washington Post said the administration was "still comfortable" with the March estimate but "may be able to beat it."

He expressed belief that the Soviets would be willing to begin negotiations and said one reason may be a Soviet desire to enter into a period of relative stability in their relations with this country while they try to get things to settle down in Poland.

In 1979, former President Jimmy Carter and Soviet leader Leonid Brezhnev agreed on a second Strategic Arms Limitation Treaty (SALT II). But that agreement was never ratified in Congress, has been all but discarded by the Reagan administration, and no new round of negotiations has yet taken its place.

Next week, Secretary of State Alexander M. Haig Jr., will meet in New York with Soviet Foreign Minister Andrei Gromyko for the first time to set up procedures for a start later this year between the two superpowers on limiting medium-range, nuclear-tipped missiles based in Europe. But as explained by the official yesterday, the administration has been trying, with some success, to develop a "conceptual theory" that would apply to negotiations on both the strategic and European-based systems.

Though these negotiations will be kept separate, at least under current thinking, he said the problems are essentially "inseparable" because there is sometimes only a thin line between strategic and so-called "theater nuclear forces" (TNF), meaning those employed only in a region such as Europe.

The official stressed that final positions on a number of points had not yet been agreed on within the administration. But he said he expected that the major initial effort of the TNF talks will be to negotiate a ceiling on Soviet SS-20 multiple-warhead missiles, more than 250 of which have already been fielded; older Soviet SS-4 and SS-5 missiles and new U.S.-built Pershing II and cruise missiles, which are still more than two years away from being deployed.

The official said the United States will probably take a position that nothing can be excluded from consideration in trying to evaluate the balance of nuclear forces between the two sides in central Europe. Thus, it is possible that the Soviets may bring up U.S. warplanes based on aircraft carriers or in England, or British and French nuclear forces, while the United States may bring up the Soviet's new Backfire bomber and other weapons.

The Backfire is one example of a weapon that falls into the so-called "grey zone" because it has enough range to reach the United States but is called a theater weapon by Moscow, so it is not clear which set of talks it will fall into. The official said the United States plans to have a single back-up group supporting both negotiations, a reflection of how closely intertwined is the whole nuclear arms field.

Haig has said the TNF talks should start between mid-November and mid-December and sources say President Reagan, two days ago, approved the nomination of Paul Nitze to head the U.S. delegation to those talks.

The official reiterated the changed assumptions that the Reagan administration brings to arms control. The United States, he said, wants simple, clear agreements that yield actual reductions in weapons to remove the threat to each others' forces and that include cooperative measures of verification, meaning an end to reliance only on what can be detected by picture-taking satellites.

The idea that Moscow shares the same view as the United States regarding what consitutes world stability has also got to be discarded. Similarly, Moscow must not be allowed to retain any edge in missile power which threatens the ability of the United States to respond and would allow the Soviets to use coercion against this country or keep the United States from using conventional forces because it was vulnerable to nuclear blackmail.

The official suggested that the president, within the next few days, will announce his decision on deploying the Air Force's new MX missile and B-1 bomber. He said the interests of the Soviets in arms control would be "absolutely zero" if the United States fails to add such weapons to its arsenal.

The presidential plan is expected to include 100 MX missiles deployed in a shell-game involving shuttling the missiles among 1,000 protective shelters, though the number of shelters could be fewer. The official claimed that administration analysis showed such a plan would keep the Soviets from developing a first-strike ability to wipe out U.S. missiles and that in time the Soviet Union might "learn the great lesson" that they will not win a costly arms race.