President Reagan informed Congress today that the United States plans shortly to conduct the first test of an antisatellite weapon against a target in space, which a spokesman said would provide an "incentive" to the Soviet Union to negotiate limits on such weapons.

Reagan promised today that the United States is bargaining "in good faith" with the Soviets to limit antisatellite weapons, or ASATs. But the White House also repeated in strong terms the administration's opposition to a Soviet proposal for a moratorium on antisatellite weapons development and testing.

The test of an antisatellite weapon fired from an F15 aircraft at an obsolete U.S. satellite is expected in September or early October, officials said. The exact date remains secret.

The first test had been delayed by technical problems with both the antisatellite weapon, called the miniature homing vehicle, and the intended target, an instrumented balloon to be inflated in space. The ASAT test vehicle is ready, but the original target is still experiencing problems, officials said. Thus, the obsolete satellite will be used instead of the balloon for the test.

The White House announcement here fit a pattern of intensified superpower jockeying in recent weeks before Reagan's summit this fall with Soviet leader Mikhail Gorbachev. Presidential spokesman Larry Speakes read to reporters a lengthy statement criticizing Soviet advances in antisatellite weapons and technology, and saying this provided a justification for the first U.S. test against a target in space.

Given the Soviet capabilities in space weapons, Speakes said, "it is disingenuous for the Soviet Union to accuse the U.S. of militarizing space."

That charge has been made repeatedly by Moscow. Two years ago, the Soviets proposed a moratorium on antisatellite weapons, which Reagan rejected, saying it would lock in a Soviet advantage in such weapons. Speakes said today a moratorium would "perpetuate" a Soviet "monopoly" on antisatellite systems. Limiting antisatellite weapons is one topic on the agenda of the stalled U.S.-Soviet nuclear weapons negotiations in Geneva and may also come up in Reagan's meeting with Gorbachev.

Although promising to negotiate toward "possible limitations" on antisatellite weapons, the White House statement today included detailed language on why the United States would not go so far as to agree to a moratorium. The statement cited "serious problems" with monitoring tests, definitions of what constitutes an antisatellite weapon, and verification.

In addition, the statement said "no arrangements or agreements" beyond those already existing on space weapons have been found to be verifiable or in the national security interests of the United States. Speakes also said the first U.S. test against a target in space is permitted by three existing treaties and the United Nations charter.

A critic of administration arms policy, Spurgeon Keeny, executive director of the Arms Control Association, said in Washington that Reagan's promises today and his rejection of a testing moratorium are contradictory.

"It's incredible that the administration can attest it is negotiating in good faith while it explicitly asserts that it is opposed to a ban on the testing and deployment of antisatellite weapons," he said.

In approving money for antisatellite weapons, Congress limited the Pentagon to three such tests. Reagan is also required to certify to Congress in advance that the United States is negotiating in good faith, and that a test is necessary to "avert clear and irrevocable harm" to U.S. national security.

Congress also required Reagan to certify that testing would not "gravely impair prospects" for future negotiations.

Reagan made these promises today. "We'll keep talking," Speakes said. But he said the Soviets already have "the only operational antisatellite system in the world" and are threatening new developments that could put the United States "further behind."

The Soviet system, made operational in 1968, is a relatively crude one compared with the sophisticated system about to be tested by the United States. The Soviet system can reach about one-third of U.S. satellites now in space, but not the high-flying early-warning and communications satellites.

Speakes said the purpose of the U.S. system is to deter the Soviets, rectify a military imbalance and provide an "incentive" for negotiations. But Keeny said, "This move on the part of the administration represents a clear escalation in antisatellite weapons since it is a substantial technological advance over the present relatively primitive Soviet system."

Speakes said that the Soviets can "destroy critical U.S. space systems," that Moscow is working on a large laser research program that could be used on antisatellite weapons later in this decade, that the Soviets could use existing satellites to manage ground forces in a conflict, and that the Soviets are targeting U.S. and allied ship movements with satellites.

The president today left his ranch near here for Los Angeles, where he and First Lady Nancy Reagan are to dine privately with old southern California friends, among other activities.