World Boxing Association light heavyweight champion Michael Spinks scored a unanimous 15-round decision tonight over World Boxing Council champion Dwight Braxton to claim the undisputed title.

The 6-foot-2 1/2 Spinks, who is almost eight inches taller than the 5-6 3/4 Braxton, successfully used his height advantage to follow a strategy of keeping Braxton at a distance, denying him the close-in position where he fights best.

"My plan was to stay completely away from Dwight," said Spinks, the younger brother of former heavyweight champion Leon Spinks. "I became like Elliot Ness. I was untouchable."

Said Braxton, "I didn't fight my fight, but believe me, I've got his number. I'll be back."

Although all three judges picked Spinks as the winner, Braxton was never seriously hurt and even in the later rounds was able to penetrate Spinks' defense to land solid punches.

But for most of the fight, Spinks kept Braxton off balance and frustrated by hammering away with jabs to the head and forehead while denying him the close-in position that he sought.

"I was in control of the fight the whole night long. I beat him with the jabs," said Spinks.

Spinks' victory brings his career record to 23-0 with 16 knockouts. He won the WBA crown July 18, 1981, defeating Eddie Mustafa Muhammad in Las Vegas, and had defended that title five times since, winning each bout by a knockout.

Braxton, now 19-2-1 with 12 knockouts, won the WBC crown Dec. 19, 1981, by beating Matthew Saad Muhammad in Atlantic City and had defended that title three times since, winning each bout by a knockout. Each fighter will receive a light heavyweight record $2.1 million for tonight's fight.

Braxton, who chased Spinks from one side of the ring to the other in some of the later rounds, called the fight "a track meet.

"He didn't win. I felt I was more aggressive. I felt I was more effective. If I did get beat I beat myself. I let him run. I didn't press the issue like I should have."

But although the margins were close, the judges were clear that Spinks was the better man tonight. Tony Castellano scored it 144-140, Frank Cappuccino had it 144-141 and Tony Perez gave Spinks the margin by 144-141.

Never at any time in the fight did Spinks appear to be in trouble, although he fell to the canvas three times in the eighth round and was the victim of a series of rapid-fire punches several times in the later rounds when Braxton was able to get through his defenses.

Referee Larry Hazzard said two of Spinks' eighth-round falls were trips and one even occurred when he had Braxton trapped in a corner and was flailing away with a variety of punches. The one fall that did count as a knockdown was a result of a punch in the stomach, Hazzard said.

Braxton, who in past fights has had a reputation for tiring in the later stages of a bout, said that was not a serious problem for him tonight. "In one or two rounds I got a little weary, but my second wind always was there."

In fact, throughout the fight Braxton appeared to show a marvelous ability to absorb punishment from the larger Spinks without its appearing to inflict significant damage. At one point, after having received a series of hard and fast punches to the body and head, he grinned and stuck his toungue out at Spinks.

An accomplished bob-and-weave fighter, Braxton was agile enough throughout the fight to send Spinks off balance by ducking and making him miss a punch, and occasionally he was able to capitalize by getting in a few hard licks of his own before Spinks could recover.

But that was never enough to compensate for the fact that Spinks kept him at a distance for most of the fight, and Braxton, who in prefight hypoberle had promised to keep so close to Spinks that "you'll think we're Siamese twins," was never able to make good on the promise.

"My fight play was to stay more on top of him, to fight on the inside," said Braxton. "There is no dog in me. The dog is in him. I'll have to go back to the drawing boards."

Said Spinks, "You never could tell whether Dwight was going to catch up with me or not. He was trying to swarm all over me, but I was catching him with the jabs. I just looked him in the eye hard enough to let him know I meant business. Maybe I hypnotized him."