China's paramount leader, Deng Xiaoping, has criticized President Reagan's Strategic Defense Initiative (SDI), describing it as a plan which would add a dangerous new dimension to the superpower arms race.

Deng's remarks on the controversial "Star Wars" program came in an interview with British publisher Robert Maxwell, chairman of the Pergamon Press and Mirror Group newspapers, last Friday at the summer resort of Beidaihe in northern China. A summary of Deng's remarks was provided by the official New China News Agency over the weekend and carried today on the front page of the English-language China Daily newspaper.

A western diplomat said today that Deng's comments constituted the first definitive, high-level rejection of the U.S. space-defense plan by the Chinese.

Some other western diplomats said recently that the Chinese had adopted a critical attitude toward SDI, partly because it helps them to maintain a public posture independent of both superpowers. But the diplomats also said that the Chinese have substantive objections to SDI, similar to those expressed by France.

"The SDI would make the Chinese nuclear deterrent a nondeterrent," said one diplomat. "And they've got to assume that the Soviets are working on it, too."

According to the news agency account of Deng's interview with Maxwell, the Chinese leader said that the Star Wars plan "must not be implemented because it would cause a qualitative change in the arms race" different in nature from simply adding a few more nuclear warheads or changing a few new types of missiles.

Previous to this, the Chinese have criticized U.S. initiative through press reports and commentaries, statements made this year at a disarmament conference in Geneva and through a Foreign Ministry spokesman here. But a western diplomat said that until recently the Chinese had been careful not to criticize the initiative through any public statements by high-level leaders.

"They said they were still thinking about it," the diplomat said.

As recently as this June, a group of Chinese diplomats and experts were reported to have sought further clarification on SDI in a meeting with Kenneth L. Adelman, director of the U.S. Arms Control and Disarmament Agency. Also in June, the Reagan administration sent a team of National Security Council experts to Peking to brief the Chinese on the U.S. program.