West German Chancellor Helmut Kohl urged the European allies of the United States today to develop a joint approach to President Reagan's Strategic Defense Initiative in order to influence future decisions on its possible deployment.

In an effort to reconcile differences in Europe as well as in his own government about the controversial program, Kohl said it was essential to keep open any commitment to build a space-based missile defense system in order to enhance prospects for early agreements at the Geneva arms talks that would involve deep cuts in strategic and medium-range nuclear weapons.

Kohl stressed that if the Geneva negotiations succeed in making drastic cuts in offensive nuclear arms in both East and West, the "deployment of space-based systems could become increasingly superfluous."

Speaking to the annual congress of his Christian Democratic Party in Essen, Kohl said, "We will continue to advocate that the Europeans develop a joint position and that they bring this to bear with our American allies."

The Reagan administration's determination to proceed with a $26 billion research program into space-based defense over the next five years has evoked mixed emotions in Europe.

While acknowledging that the so-called Star Wars project was probably a key factor in bringing Moscow back to the arms control talks, the European allies have expressed concern that the program might develop such momentum that antimissile technology soon could be deployed that would undercut western deterrent strategy.

Allied leaders have ostensibly backed U.S. research to counter the Soviet Union's own space defense experiments, but they have also warned that an uncontrolled spiral in offensive and defensive weapons systems could jeopardize the North Atlantic Treaty Organization's cohesion as well as their own national interests.

Britain and France are worried that a leap toward new antimissile defenses or more effective nuclear weaponry could render obsolete their small nuclear-missile forces, based on land and sea, that are now poised to undergo expensive modernization.

West Germany is concerned that vast infusions of money into SDI eventually would divert resources from NATO forces in Central Europe, thus upsetting the conventional balance.

West German Foreign Minister Hans-Dietrich Genscher, echoing reservations voiced by British Foreign Secretary Geoffrey Howe last week, warned Monday that the United States and its European allies "must not be decoupled through technological innovation."

"Absolutely nothing must be allowed to endanger the high moral goal" of deterrence, Genscher said. "Every new development must therefore be examined to see whether it brings us closer to the goal of preventing war."

Despite Foreign Ministry denials of a rift between himself and Genscher on the potential repercussions of SDI, Kohl appears to have adopted a more sanguine perspective toward the project. He has endorsed research into space-based defense but wants the European allies to share in the economic and technological spinoffs that emerge from the U.S. program, according to his aides.

Senior chancellery officials said Kohl's call for a joint European approach to SDI reflected his conviction that the allies would gain a greater voice in ultimate decisions on deploying space-based systems if they became actively involved in the research phase.

Officials in Bonn's defense and research ministries also have argued that since the Reagan administration appears determined to press ahead with its massive research program, West Germany should seek to reap economic benefits by proposing its own contributions to the project.

Such tasks might include advanced research in optic lasers, space sensors and other fields that would serve to bolster Europe's high-technology industries.

Kohl is also persuaded, a top adviser said, that the Geneva arms talks will secure deep cuts in the superpowers' nuclear arsenals only if Moscow is prodded toward an agreement by the risk that SDI will yield enormous technological advantages for the West.

Kohl appealed in his speech for both the United States and the Soviet Union to channel their mutual research efforts into antimissile systems to lead to cooperative agreements that would strengthen the 1972 antiballistic missile treaty and forestall an arms race in space.

He said the opening of the Geneva arms talks last week augured well for a more enduring era of detente but contended that this could happen only if the smaller countries in Eastern and Western Europe were permitted to develop their own independent relations.

In a clear admonition to Moscow not to interfere with Bonn's efforts to promote better contacts with Eastern Europe, Kohl said, "Whoever tries to hinder this dialogue will raise questions about the credibility of his political intentions."

Kohl also urged the new Soviet leader, Mikhail Gorbachev, to fulfill promises he made during a meeting in Moscow last week for an imminent improvement in Soviet-West German relations.