California's 75-year-old junior senator, S. I. Hayakawa, shocked a state Republican convention today by announcing he had changed his mind and would not seek reelection.

Hayakawa, who had gained a reputation for falling asleep in meetings and stood low in the polls, indicated that he had decided he could not carry on a vigorous campaign against seven primary opponents and still see to his Senate duties this year.

The announcement took even Hayakawa's supporters at the convention in Monterey by surprise, for the flamboyant former San Francisco State University president had stubbornly resisted veiled offers of ambassadorship or academic posts if he would leave the race in favor of younger, more vigorous candidates.

"I make this choice without urging or pressure from anyone except my own internal imperative to turn in a record of solid legislative achievement as my small contribution to the history of this state," Hayakawa told the more than 1,000 gathered at the convention.

His press aide, Sandra Conlan, said afterwards the senator had discussed the "hard facts of life" with Senate leaders and had spoken to President Reagan Thursday.

She said Hayakawa felt he could win the race, but in the last statewide poll he was in fourth place with 15 percent against primary opponents Rep. Barry M. Goldwater Jr., 22 percent, San Diego Mayor Pete Wilson, 19 percent, and Rep. Paul N. (Pete) McCloskey, 18 percent.

Rumors began to sweep the convention soon after it was learned that Hayakawa would address the state Republicans this morning, rather than at his scheduled time this afternoon, when all the GOP primary candidates would appear. Also in the race for the Republican nomination are the president's daughter, Maureen Reagan, Rep. Robert K. Dornan, state Sen. John Schmitz and businessman and former law school dean Ted Bruinsma.

The winner of the Republican primary is expected to face California Gov. Edmund G. Brown Jr. in the general election. Although recent polls have shown the top Republican contenders beating Brown, the most recent Los Angeles Times poll showed that Brown would beat Hayakawa 40 percent to 32 percent.

The large field in the primary reflected the general feeling that Hayakawa could not win. His campaign manager, Ron Smith, had quit earlier with a sharp critique of the ineffectiveness of the senator's staff. Hayakawa had spent about $250,000 in his campaign so far and had been addressing large student groups up and down the state.

In his speech today, Hayakawa defended his Senate record at length, pointing to his efforts to improve the chances for teen-age employment throughout the country. He said he suffered from tackling the most difficult issues. In the past, he had provoked controversy by off-the-cuff remarks debunking complaints of Japanese-Americans about treatment in World War II relocation camps and a comment that rising gas prices wouldn't affect the poor because they didn't have jobs and didn't need to drive.

During his 20-minute speech, Hayakawa said he planned to resume a newspaper column and go on a lecture tour after he finishes his term.

The short, bubbly senator, for most of his career known as an academic expert on semantics, became prominent in California after he strongly resisted student demonstrations at San Francisco State a decade ago.

In 1976 he ran against the then-incumbent Democratic senator John Tunney and won in an upset.