Soviet negotiators last week made a new proposal at the U.S.-Soviet negotiations on long-range nuclear weapons that would limit each side to about 1,100 multiple-warhead missiles and bombers, but would let Moscow keep all the big missiles that most threaten this country, according to U.S. officials.

The officials said the continued unwillingness of the Soviets to reduce their force of about 640 big SS18 and SS19 missiles means that the new proposal still fails to address the weapons about which the United States is most concerned.

But the new proposal may represent the most important move by the Soviets in a pattern of stepped-up activity in recent weeks at the Strategic Arms Reduction Talks (START) in Geneva. Although U.S. officials describe this activity generally as "cosmetic" rather than serious so far, they are still analyzing this new move to determine whether it may at least represent a signal that more serious bargaining can take place.

The new Soviet proposal indicates that Moscow would agree to lower overall limits on multiple-warhead missiles, or MIRVs, than it agreed to in the 1979 U.S.-Soviet SALT II strategic arms limitation agreement, which was never ratified by the United States.

But the key stumbling block remains the big Soviet missiles. The United States claims that the 308 SS18s, which can carry 10 warheads each, and the approximately 330 six-warhead SS19s theoretically are capable of wiping out U.S. land-based missiles in a first strike. The Reagan administration insists that this threat be removed.

Soon after the START negotiations began in June, 1982, the Soviets proposed an overall limit of 1,800 on what Moscow calls "strategic nuclear delivery vehicles," meaning ocean-spanning missiles and bombers.

U.S. officials said the Soviets did not modify that until about three weeks ago, when they dropped demands that the United States sharply limit its submarine-based missile force and the range of cruise missiles carried aboard American bombers. U.S. officials said the original Soviet demands were unacceptable but that their removal at least showed some movement.

Last week, the Soviets proposed a sub-ceiling of 1,100 on MIRVed missiles and bombers within the overall ceiling of 1,800 strategic nuclear delivery vehicles, according to the officials.

The Soviets were reported to have further divided this into three separate limits: one for bombers equipped with long-range cruise missiles, another for a combined total of all submarine-based and land-based MIRVed missiles and a third for land-based MIRVed missiles, which are the most accurate and threatening. The Soviets, however, placed no restriction on which specific missiles could be fielded within the general categories.

Officials suggested that the Soviet proposal would allow about 700 land-based mutliple-warhead missiles, which would mean all the existing SS18s and SS19s, plus some four-warhead SS17s, could be retained or replaced by more modern versions. It also reportedly would allow for about 150 missile-equipped bombers.

The new Soviet proposal reportedly follows closely the formula that both countries agreed to in the earlier SALT II pact, which they have pledged to observe even though it was not ratified by this country. The Soviets have frequently urged that SALT II be ratified.

Under SALT II, both sides would have frozen their total missile and bomber force at 2,250, with a sub-limit of 1,320 on MIRVed missiles plus missile-equipped bombers, with a maximum of 820 of those MIRVed missiles on land.

The Soviets are now suggesting 1,800 total weapons with a sub-limit of 1,100 MIRVs, about 700 of those on land, sources said.

The United States now has about 1,600 long-range missiles and 410 bombers, while Moscow is credited with about 2,350 missiles and 350 bombers.

The original U.S. proposal at START called for each side to have only 850 missiles. But Reagan recently announced that he would increase that, if necessary, to a level which officials said was about 1,100-1,200. This would be a step toward the Soviet position of 1,800 weapons.

The Soviet figure, however, also includes bombers, so the two sides do not appear too far apart now on the overall number of missiles, although the type of those weapons remains the source of strong differences.