The step-by-step search for a defense in space against Soviet ballistic missiles is on track and the first phase of the "Star Wars" program will be technically deployable between 1995 and 2000, a senior U.S. negotiator said Thursday.

In a prepared speech to a major defense contractors group, Edward L. Rowny said research "is not only alive but healthy and thriving" in such areas as developing technology that can discriminate between Soviet decoys and nuclear warheads targeted on the United States.Rowny, who advises President Reagan and Secretary of State George P. Shultz, disputed published reports that the administration was scaling back the program, known formally as the Strategic Defense Initiative.

Recently, he said, six key SDI technologies were approved by Defense Secretary Frank C. Carlucci for demonstration tests to validate they would help shield the country from Soviet ballistic missile attack.

He acknowledged that an unpublished report by Congress' Office of Technology Assessment last July identified some "gaps" in Star Wars technology. But Rowny, a former Army lieutenant general who has participated in negotiations for 20 years, said development has moved quickly in the nine months since the report was submitted to the Pentagon.

As an example, he cited an experiment known as Delta 181 that discriminated between decoys and warheads.

Rowny, addressing a meeting of the American Defense Preparedness Association, urged the contractors "to get the truth out about SDI."

He criticized reports in The Washington Post that the Pentagon was sharply scaling back its efforts to fulfill President Reagan's dream of rendering ballistic missiles "impotent and obsolete" with a space-based defense against them.

The first phase of the program would use kinetic energy and other futuristic technology to provide a partial defense against nuclear missiles.

Rowny said deployment would "greatly complicate" the ability of Soviet military planners to executive a successful first strike of ballistic nuclear missiles against the United States.

Critics contend the program is fanciful, extremely expensive and a major stumbling block in negotiations with the Soviet Union to reduce long-range nuclear weapons by 30 percent to 50 percent.