The Defense Department is planning slight modifications in possible components or testing methods for a new space-based missile defense system so tests can be conducted without violating the 1972 Antiballistic Missile Treaty with the Soviet Union.

The Pentagon plan, revealed in a report sent to Congress last week on the strategic defense initiative (SDI), is likely to be controversial, since the ABM treaty specifically outlaws development, testing or deployment of any "component" of a space-based missile defense.

Pentagon lawyers, however, have interpreted the treaty language as permitting activity "short of field testing of a prototype ABM system or component." The Pentagon's legal interpretation exploits the fact that the ABM treaty did not include a precise definition of "component."

According to the plan outlined in the new report, component parts that may someday be part of a missile defense system could be tested by modifying them so that, in the exact form tested, they would not be usable in a missile defense system.

This approach quickly drew criticism from opponents of the Reagan administration's SDI program, also known as "Star Wars."

One congressional aide said yesterday that the report showed the Pentagon has used "creative lawyering" to justify its test plans. He added that the Pentagon was taking advantage of loopholes as the Soviets have done in this and other treaties.

Paul C. Warnke, director of the Arms Control and Disarmament Agency in the Carter administration and a lawyer himself, called the Pentagon's approach "a total fraud."

"This is the kind of reasoning that would bring the total arms control business into disrepute," Warnke said. "If this were a Soviet report, we would decide there was no reason to continue negotiating with them because it would be obvious they weren't serious about limitations."

The testing techniques proposed in the Pentagon report, which was released last Thursday, include some specific programs, identified as being in a gray area, where there will be "field testing of devices that are not ABM components or prototypes of ABM components." Among them:

* The SDI program is developing a non-nuclear armed device that would home in on a missile or warhead and kill it. The report says using such a device against "strategic ballistic missiles or their elements in flight would clearly not be permitted" under the treaty. Therefore, the report says, the Pentagon has decided to demonstrate its capabilities "against antisatellite interceptors," which will "also permit a decision to be made on the applicability of more advanced technology for ABM purposes."

* The space-based high-speed gun known as a railgun, which also is being developed for use against ballistic missiles, will be tested "to defend against antisatellite interceptors." Here, too, such a test will enable the Pentagon to determine "applicability of more advanced technology for ABM purposes."

* Because a ground-based laser fired into the atmosphere with power and beam quality enabling it to knock out a warhead would be a violation of the treaty (unless it is first discussed with the Soviets), the SDI laser will be tested "below the power level and beam quality required for a ground-based laser ABM weapon," the report said. In addition, it will not be tested against "ballistic missiles or their elements in flight."

* The boost-phase surveillance and tracking system is a satellite-development program that is to provide both early warning and tracking of Soviet missiles in the first minutes after launch. The satellite will read the exhaust plumes of Soviet rockets and send the data down to Earth. To prevent the satellite from being a component, the report says, the "experimental device will be limited in capability." It may, for example, "measure the signatures of booster plumes, but not in real time."

* The space-based surveillance and tracking system (SSTS) is designed to track objects in space during the mid-flight of a missile attack. This device must distinguish decoy warheads from real ones and be able to track both types. The test device, according to the report, will be able to collect and track "a number of space objects." But to stay within the treaty limits, its "capabilities . . . will be significantly less than those necessary to achieve ABM performance levels or substitute for an ABM component," the study says.

* The airborne optical adjunct is a Boeing 767 airplane outfitted with sensors which will fly high enough to provide identification and tracking information on warheads before they reenter Earth's atmosphere. This system will not be able to substitute "for an ABM component due to . . . limitations" on the plane and the equipment it carries.

Using the test guidelines, the report said, "research necessary to support a decision on the potential utility of the SDI technology can be conducted in accordance with U.S. treaty obligations."

Critics of the program have argued that SDI tests would violate the treaty before enough information was available in the next seven years for a Reagan successor to make an informed presidential decision on whether to go ahead with development.

The report acknowledges that the goal of the research is to establish the feasibility of a space-based missile defense system that, if deployed, would violate the ABM treaty unless the Soviets agreed to it beforehand.

The report is likely to be discussed in current arms negotiations at Geneva, where a Soviet violation of the ABM treaty, through the building of a radar facility in central Siberia, has been a focus of American attention.

Warnke said that under the Pentagon report's interpretation "there would be no violation yet" at the Siberian site, as contended by Reagan and other officials, because the facility has not been activated.