SAN SALVADOR, El Salvador (AP) _ Roberto d'Aubuisson, the leader the arch-conservative opposition party ARENA, has stepped down in an apparent effort to prevent his growing unpopularity from hurting the party, officials said.

A convention of about 800 members of the Republican National Alliance, or ARENA, on Sunday replaced d'Aubuisson with Alfredo Cristiani, a little-known coffee grower said to be about 40 years old.

D'Aubuisson, a 40-year-old former National Guard major, headed ARENA since it was founded four years ago. He has been linked in many accounts to El Salvador's rightist death squads, but he consistently denies those charges.

Many ARENA members said d'Aubuisson's flamboyant rhetoric had done much to alienate voters.

They said that while the positions of their party were acceptable to voters, d'Aubuisson's public statements seemed extremist even to some of his avid supporters.

ARENA's newly named executive director, Ricardo Alvarenga Valdivisio, explained the change in party leadership by saying, ''Each time they attacked d'Aubuisson they attacked the party, and what we want is that the party not be a person, but an entity.''

Convention delegates voted to award d'Aubuisson the title ''honorary ARENA president for life,'' but the position appeared to carry no power. The decision to choose a new party leader apparently was made jointly by d'Aubuisson and ARENA.

The former leader retained his job as a member of the National Assembly from ARENA. Last year d'Aubuisson was ARENA's candidate for president, but he was defeated by Jose Napoleon Duarte, the Christian Democrat.

Many ARENA backers have switched to a new party, Patria Libre, or Free Fatherland, founded by businessman Hugo Barrera, d'Aubuisson's disenchanted running-mate in the 1984 elections.

The Patria Libre is promoted by Barrera and other backers as an attempt to clense the image of the right, something Barrera has hinted widely could not be done through ARENA with d'Aubuisson at its head.

The extreme right is widely blamed for thousands of political killings in El Salvador, including that of the Rev. Oscar Romero, archbishop of San Salvador, in 1980.

The killings coincide with a civil war that has pitted leftist guerrillas against Duarte's U.S.-backed government, and has resulted in about 60,000 deaths.

At Sunday's convention, Wanda de Silvia, the leader of ARENA's Feminine Front, told delegates: ''We must give honors to this man who put up a wall to the communism that invades us now.''

Until now ARENA had basically been a vehicle for d'Aubuisson. The party had led a rightist bloc in the National Assembly that had been able to stymie Duarte's moderate reform efforts.

This spring, Durate's Christian Democrats won an absolute majority in the 60-member assembly despite ARENA's alliance with another conservative party, the National Conciliation Party.

D'Aubuisson had pushed the alliance against the advice of many former ARENA members, and many in his party blamed the alliance for the eventual loss of conservative power in the National Assembly.