Steve Cauthen, who won his greatest fame riding Affirmed to his sweep of the Triple Crown last wpring, was fired today as the colt's jockey.

Trainfer Laz Barrera announced that Laffit Pincay Jr. will handle Affirmed in the $200,000 Charles H. Strub Stakes at Smanta Anita next Sunday.

"This is one of the hardest decisions I ever had to make in my life," Barrera said. "It was very, very painful. But I had no other choice."

Of course, Barrera did have choices. He could have sacked Chuthen with justification at other times during the past year, but instead he chose to kick the teen-ager when he was down, mired in a 105-race losing streak. He may be using the jockey as a scapegoat, blaming him for Affirmed's two recent losses in California when the fault lies with the horse or his trainer.

There were strains in the Barrera-Cauthen relationship even at the peak of their success. Barrera had done a brilliant job training Affirmed through the Kentucky Derby, the Preakness and the Belmont Stakes, but his achievements had been overshadowed because of the media's love for Cauthen.

Once, after the Preakness, he let his resentment erupt. "Steve Cauthen thinks he's the only hero in this damn game," Barrera snapped.

It would have been unthinkable to break up a winning combination when Cauthen was riding so brilliantly. But after the Triple Crown series, New York race-goers started wondering if they were seeing the same Cauthen who had once seemed so flawless. The youngster was not winning races with his usual frequency, and he was guilty of frequent errors and lapses in judgment.Even Affirmed was not immune to his mistakes.

In the Marlboro Cup at Belmont Park, Cauthen badly misjudged the early pace, letting Seattle Slew steal off to an uncontested three-length lead in the first quarter-mile. By the time the jockey got Affirmed into gear, it was too late. Barrera blasted Cauthen publicy, blaming him for the defeat.

When Cauthen and Affirmed went to California this winter, their problems multiplied. The jockey who had once grabbed headlines for winning five or six races a day now was the subject of daily stories that chronicled his losing streak and specullated about its cause. Was it women? Bad mounts? The after-effects of two accidents on the track?

Whatever the reason, Cauthen has not won a race since Jan. 1, and two of his 105 consecutive defeats came aboard Affirmed. In the Malibu Stakes, he got the colt trapped on the rail and was beaten by Little Reb and Radar Ahead. The loss, however, may not have been solely his fault, because two weeks later in the San Fernando Stakes, Affirmed again was beaten by Radar Ahead, with no excuses.

Unlike many horses who blossom between their 3- and 4-year-old seasons, Affirmed seems to have wilted. The reasons are not difficult to understand. The colt had an unusually tough campaign during 1978, from his early season prep races in California to his year-end battles with Seattle Slew. Ordinarily, a thoroughbred would be given a long lest after such exertions and be returned to competition in May or June.

But Affirmed never got to enjoy such a period of recuperation and was rushed back into competition Jan. 7. It is dangerous to second-guess Barrera, who is the best trainer in America, but may have misjudged his horse. If that is the case, Cauthen is now helping to pay for his mistake.