In the secret diplomacy leading up to next week's summit meeting, the Soviet Union has proposed that each nuclear superpower quickly reduce its land-based intercontinental missiles by 200 to 300 to demonstrate their good faith in the arms negotiations, administration officials said yesterday.

The unannounced Soviet proposal was first made in the Geneva nuclear and space arms negotiations in early October, and was repeated last week in Moscow to Secretary of State George P. Shultz, the officials said.

While the United States has neither rejected nor accepted this Soviet offer, some administration officials have described it as an attempt to put the United States on the defensive at little cost to Soviet military strength.

U.S. negotiators have so far been unable to learn the details of the cutback the Soviets proposed, officials said.

If it applies to all types of land-based intercontinental missiles without restriction, they said, the Soviets could simply eliminate 200 or 300 of their oldest and least effective missiles, which would have been retired four years ago if the SALT II treaty, signed in 1979, had ever been ratified. But the United States might have to make most of its cutback from its active Minuteman missile force, the backbone of the U.S. land-based deterrent.

The Soviet Union is believed to have about 1,459 land-based intercontinental ballistic missile launchers, or missiles, while the United States has about 1,032 comparable weapons. The Soviet proposal is for the reduction to be made in launchers rather than in warheads, which is the unit of account preferred by the United States.

There is speculation in some quarters here that the Soviet proposal for this immediate reduction in missiles could be unveiled next week as a propaganda ploy when President Reagan meets Soviet leader Mikhail Gorbachev in Geneva.

At the White House yesterday, a senior administration official told reporters that the summit meeting would not produce any immediate results in the field of arms control. Rather, he said, "the president believes that results in arms control will only be expressed in the months that follow" the summit, and in the subsequent pace of superpower negotiations in Geneva. The summit may produce a "joint commitment to accelerate progress" at the bargaining table, he added.

He said Reagan intends to propose expanded contacts in cultural and other areas with the Soviets in a nationally televised address tonight about the summit.

Late last night, State Department officials who asked not to be identified told the Associated Press that the United States and the Soviet Union have reached the basis of an agreement to broaden cultural exchanges. The agreement would provide for Soviet and American students to visit each other's countries and for faculty members, scientists and journalists to have reciprocal visits as well, the AP said.

Officials also said last night that the two countries were seeking agreement on a joint statement regarding efforts to halt the proliferation of chemical weapons. The United States has been increasingly concerned with the spread of chemical warfare, especially since the use of poison gas in the Iranian-Iraqi war.

Gorbachev, in a visit to Paris in October, gave the first indication that the Soviets share this concern, announcing that the Soviet Union is ready to cooperate in a nonproliferation effort against chemical weapons.

The two countries are also expected to make a joint statement in Geneva of their continuing and perhaps stepped-up efforts to halt the spread of nuclear weapons.

They have been actively cooperating in this field for more than a decade.

At a news conference yesterday, House Democratic leaders declared strong support for Reagan at the summit and urged him to reach agreement with Gorbachev on a broad array of issues, from nuclear disarmament to human rights.

The Democrats also predicted that Reagan would score some significant successes in the meetings.

Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) pledged "a bipartisan willingness to implement any agreement" reached in Geneva. "When President Reagan meets with Mr. Gorbachev next week he deserves the support of all Americans, regardless of party or philosophy," he said.

O'Neill added that his "political antenna" tells him that Reagan and his aides "will have some kind of agreement, that they will not come back empty-handed."

In another arms control development yesterday, U.S. officials revealed that the Reagan administration, in a previously unreported section of its recent Geneva arms proposal, is willing to eliminate most of the Pershing II intermediate-range missiles now being deployed in Western Europe.

The U.S. proposal calls for a limit on European-based intermediate-range ballistic missiles of 140 launchers on each side, with a warhead limit of about 420 to 450 on each side.

The Soviet side would be expected to deploy all its 140 launchers in SS20 missiles with three warheads each, for a total of 420 warheads.

The U.S. side, officials said, contemplates as few as 36 single-warhead Pershing II missiles plus 104 ground-launched cruise missile launchers with four single-warhead missiles on each launcher. This would bring the U.S. Euromissile total to 452 warheads.

A total of 108 U.S. Pershing II missiles will have been deployed in West Germany by Dec. 31, most of which are operational now.

Elimination of all the Pershing IIs was resisted by the Pentagon in mid-1982 when it was proposed by Ambassador Paul H. Nitze as part of his "walk in the woods" formula in the Euromissiles negotiation, which the Soviets also rejected.

But the current plan to reduce the Pershing IIs to as few as 36 missiles has reportedly been accepted by the Joint Chiefs of Staff, on the condition that the Soviets accept all the U.S. conditions attached to it.