U.S. negotiators in Geneva introduced a four-page draft treaty on ballistic missile defenses yesterday as President Reagan, responding to concerns from the Joint Chiefs of Staff, instructed his negotiators to ensure that the Soviets "understand and accept what is meant by the U.S. interpretation" of the draft treaty, U.S. officials said.

The new U.S. draft incorporates ambiguous language first used at the December summit in Washington that says each superpower should adhere for a specified period to the 1972 Antiballistic Missile (ABM) Treaty "as signed," and conduct testing of space-based defensive weapons "as required." The Joint Chiefs complained that this formulation was too vague, leading to Reagan's new instructions to his negotiators to win Soviet approval for the U.S. interpretation of the language, and thus of the administration's understanding of the ABM Treaty.

White House spokesman Marlin Fitzwater reaffirmed yesterday at a briefing for reporters that the administration believes the correct interpretation of the ABM Treaty is a "broad," or permissive, interpretation allowing realistic tests in space of technology developed under the Strategic Defense Initiative (SDI), or "Star Wars," program.

The Soviets, along with a majority in Congress and many former U.S. ABM Treaty negotiators, have argued that the pact bars such tests. As a result, several U.S. officials said they expect the Soviets to reject U.S. demands that they accept the "broad" interpretation.

At the same briefing, Fitzwater criticized a report in yesterday's Washington Post on the Joint Chiefs' complaints and the president's new negotiating instructions. "I have seldom seen a story with so many wrong angles," Fitzwater said. He did not return phone calls from The Post seeking clarification of this remark.

Fitzwater confirmed that the Joint Chiefs had written the White House, in Fitzwater's words, "urging that . . . the negotiators be instructed to reach an understanding and a definition of what we mean by {the} language" agreed to by Reagan and Soviet leader Mikhail Gorbachev at the summit.

But he denied that Reagan's instructions to U.S. negotiators demanded Soviet acquiescence to the "broad" interpretation, as The Post reported. The instructions did not "focus" on the "broad or narrow interpretation," Fitzwater said, but instead on the summit agreement to allow testing as required by U.S. and Soviet missile defense programs.

"Broad versus narrow" would not necessarily be a subject of negotiations, Fitzwater said. Another senior official said the terms broad and narrow would not be used, but that the United States would press the Soviets to accept "our" interpretation of the ABM Treaty. The classified cable containing Reagan's new instructions sent to Geneva referred to the need to win Soviet acceptance of "the U.S. interpretation."

Other U.S. officials said the chiefs expressed their concern in a letter to national security adviser Colin F. Powell that was signed by their chairman, Adm. William J. Crowe Jr. The chiefs' concern was that the formula's ambiguity would leave room for the Soviets to interpret the missile-defense research constraints as they wished, and possibly to allege U.S. violations of their interpretation as a pretext for abrogating a future pact to reduce strategic offensive arms.

Fitzwater said The Post reported incorrectly that the chiefs urged in their letter to the White House that the administration reject the ambiguous summit formula on missile defense tests and instead choose one of two alternatives.

The Post reported one alternative suggested by the chiefs was to reach agreement with the Soviets on a detailed list of acceptable space weapons tests. Another suggestion was that the administration agree not to withdraw from the ABM Treaty for five years, instead of seven as previously proposed. The chiefs' rationale was that no disputed space tests were likely to occur within this period, and afterward both sides would clearly be free to conduct unlimited tests, officials said.

U.S. officials said yesterday that the proposals were agreed to by the chiefs and advanced through other means, but not urged in the Crowe letter.