Defense Department officials plan to continue development of a $4 billion U.S. antisatellite weapon despite a week-old law that bans tests against an object in space as long as the Soviets do not test such a system, according to a Pentagon official.

One possibility under study is to fire a test weapon against "a point in space" rather than at the two, $20 million targets that were put into orbit Dec. 12, the official said.

The Air Force "won't do anything in direct violation" of the new congressional language that was attached to the omnibus spending bill and signed into law by President Reagan last Wednesday, the official said. But, he added, "We will find a way to go ahead."

The antisatellite (ASAT) weapon is launched from an F15 fighter and guides itself into the path of a target satellite, destroying it on impact. The development program was initiated in 1977 by then-President Jimmy Carter as a bargaining chip in negotiations with the Soviets to bar such weapons. When those talks failed, the Reagan administration pushed development of the system because the Soviets had a rudimentary antisatellite weapon.

In 1983, the Soviets announced a moratorium on testing their ASAT system and called on the United States to follow suit. President Reagan refused, arguing that the United States had to develop its own system. U.S. critics of that decision said the more advanced American weapon could trigger an antisatellite arms race.

Congressional opponents of the antisatellite system hailed approval of the House-Senate language banning tests against targets in space as a major arms-control step. Rep. George E. Brown Jr. (D-Calif.) said that without the ban the Air Force would have conducted two tests next year against targets.

"With two more successful tests," Brown said, "the Air Force could have declared the system operational and begun the process of full deployment."

Congress last week also removed $98 million the administration had sought to begin procurement of the antisatellite weapon this year. Brown said recently that he planned to seek an end to the system next year. "If they want to go after satellites," he said, "they should go for a modern laser system."

One of Defense Secretary Caspar W. Weinberger's top assistants said yesterday, "The congressional ban must be undone." He called the antisatellite program one of the administration's most important efforts that "must be continued."

A Pentagon spokesman said continuing the program "will keep the Soviets' feet to the fire" in arms-control talks.

The project has been controversial with the uniformed services, in part because it is not effective against advanced Soviet satellites, and in part because it has cost so much.

To date, some $1.2 billion has been spent on its development, according to Pentagon sources. That has bought two test weapons, one of which was fired against a point in space in 1984, and the other against an old satellite in September.

There also has been funding of nine target missiles, 13 test weapons and modifications for F15 fighters used for testing.

This year, Congress split on limiting the program. The authorization bill approved permitted two tests. When the funding bill came up, however, the House voted to prohibit tests against a target in space "until the president certifies that the Soviet Union has conducted, after Oct. 5, 1985, a test against an object in space of a dedicated antisatellite weapon."

Pentagon lawyers determined that they could finance the two tests the Air Force planned for next year by using 1985 funds. They suggested that Senate conferees accept the House language, one participant in the conference recalled, because it limited spending of only 1986 funds.

However, this source said, Sen. Ted Stevens (R-Alaska), chairman of the Senate conferees on defense, warned House members of the Pentagon plan and they, in turn, added language that prohibited use of past year funds for the tests.