West Germany declared its support for research into space-based defense today but put off any decision to participate in the Reagan administration's Strategic Defense Initiative until later this year.

In a policy declaration, Chancellor Helmut Kohl's government said the SDI research program was "justified" by the Soviet Union's own space defense experiments and its possession of an antiballistic missile system that is now being modernized.

But senior West German officials on an interagency task force studying SDI said the Bonn government would not make any commitment to join the program until it learned more about the nature of U.S. research plans and received guarantees that all research and technology will be fully shared.

Lt. Gen. James Abrahamson, head of the SDI project, met with Kohl in Bonn today to discuss the space research program, but their talks were not expected to alter West Germany's intention to delay a decision on its possible contribution, officials said.

Defense Secretary Caspar W. Weinberger, addressing the North Atlantic Treaty Organization's nuclear planning group in Luxembourg Tuesday, invited American allies to take part in the research program. He requested that they respond within 60 days.

But officials here said West Germany does not feel obliged to meet this deadline, particularly because Kohl wants to speak at length with President Reagan about the SDI project when he visits Bonn in early May.

The officials said that the United States was not expected to finish its preliminary studies outlining the five-year, $26 billion program until fall. Thus, West Germany could not decide whether it will participate until it knows what the SDI project will entail.

Bonn insists that before joining the program, it must have assurances that West German firms will have full access to all information and new technologies that emerge during the research.

"If cooperation is a one-way street with us providing the components but without getting in on the final results, there is no great interest on our part," a senior official said. "We want a full exchange of technology."

Bonn's Defense Ministry already has specified several fields where the country's technological expertise could be applied in the SDI program. West Germany is considered a leading force in optical sensor research and its companies are also strong in high-frequency data processing, ultralight materials and systems components for extremely fast rockets.

While eager to develop such industries by pooling data and research with the United States, Bonn stipulated today that its participation also would depend on whether the proposed space defense system would shield Europe, and not just the United States, from Soviet missiles.

The policy statement said NATO'S political and strategic unity must be preserved, and it emphasized that "the security of Europe must not be decoupled from that of the United States."

Besides his meeting with Kohl, Abrahamson and accompanying military advisers gave a 90-minute classified intelligence briefing to members of the West German parliament's defense committee on the Soviet antimissile defense system.