White House science adviser George A. Keyworth II, one of the administration's chief promoters of President Reagan's Strategic Defense Initiative, said yesterday that he will resign and form a company to help corporations build up intelligence-gathering systems.

Keyworth said he has done what he could in four years and thought it wise to leave the job at "a happy point."

He said he believes that the missile defense research program is well under way, that science funding has been boosted and that cooperation between industry and universities has been pushed along.

Keyworth said he was not leaving because of any conflict in the administration.

Asked why he wanted to leave when things were going well, he said, "I have been here pushing five years now. I think the president needs some fresh blood. When I walk into a room everybody knows what I'm going to say.

"Also, I simply want to leave," he said. "I am beginning to be envious. The real action is out there in industries that are trying to find a new competitive spirit, motivated by the hot breath of competition on their necks. That's very exciting." He said he reads with envy of the corporate battles of such companies as IBM against foreign challenges. "I want some," he said.

Keyworth said he plans to start a new business with Herbert Meyer, former special assistant to Central Intelligence Agency Director William J. Casey, and now vice chairman of the National Intelligence Council. The council drafts intelligence estimates from data developed by half a dozen agencies.

Keyworth's new business will offer to set up intelligence-gathering systems for companies to gather information about potential markets. The company will not gather intelligence, but will show clients how to find and analyze political, economic, cultural and other information for use in foreign markets.

The accomplishments Keyworth, 46, said he was proudest of are his work for the "Star Wars" research program, his work to bring greater cooperation between industry and universities and his role in building science budgets.

Perhaps the two most controversial aspects of Keyworth's tenure have been his strong advocacy of the strategic defense initiative and his reversal of the historical role of the president's science adviser. From the day he took the job, Keyworth said he would not be a conduit into the White House for American science as previous advisers have.

Instead, he would be part of the White House "team." He also set up the White House Science Council, a low-profile body of scientists offering information to the administration. It was not a forum for debate like the previous body, the Science Advisory Committee.

Keyworth said he did not tell anyone in government of his leaving until Wednesday, when he informed White House chief of staff Donald T. Regan. He will leave office by Dec. 31.

"He is going to be missed," said William Carey, executive officer of the American Association for the Advancement of Science, largest scientific body in the United States. "Keyworth has developed over his five years in the job into an effective and sometimes aggressive advocate of fundamental research."

Other leaders in science have said Keyworth's ability to keep science funding high while other budgets were being cut was his chief accomplishment.

But Carey said Keyworth has recently expressed apprehension about the prospects for keeping up government support for science.

"He has been trying to warn the scientific community that there are dark days ahead," Carey said. "I can't look into his mind, but I think he saw a period of budgetary siege approaching and that he might not be able to see effectively to protection of research.

"Perhaps we could have been helped if Keyworth stayed on," Carey said. He noted that it will be a difficult time for a successor now, just before budget deficits lead to cutting of "discretionary" budgets such as those for basic research