Rainer Barzel, the speaker of the West German parliament, resigned today after being accused of accepting more than $500,000 from the Flick industrial concern to step down in 1973 as chairman of the Christian Democratic Union in favor of the current chancellor, Helmut Kohl.

The decision was announced today by Alfred Dregger, the party's parliamentary floor leader, who said Barzel was quitting his post because "the political and psychological pressure against him had become unbearable."

Barzel has described the charges made against him last week as "infamous." He has insisted that he never took any money from Flick, one of West Germany's biggest holding companies.

But a former Flick manager, Guenter Max Paefgen, testified today that the company decided to give a contract to an obscure Frankfurt law firm, Albert Paul, only after it hired Barzel. Paefgen appeared before a parliamentary commission investigating alleged attempts by Flick to bribe politicians and parties to endorse tax exemptions it sought in the 1970s.

The inquiry into Flick's financial involvement in West German politics has aroused a stormy debate about the vulnerability of the country's democratic institutions to bribery and influence-peddling.

Polls show that the public is increasingly concerned about the dominant role of big business in bankrolling politicians and their parties. West German politicians, unlike American, generally are shielded from public scrutiny of their property and tax records.

The opposition Social Democrats welcomed Barzel's resignation and accused the Kohl government of being incapable of overcoming "the crisis of trust that has shaken up our democracy."

Barzel is the second leading politician in Kohl's center-right ruling coalition to succumb to the "Flick affair." Four months ago, Economics Minister Otto Lambsdorff resigned after learning that he would stand trial on charges of taking payments from Flick in exchange for granting of favorable tax treatment.

Barzel came under fire after the weekly Der Spiegel reported that Flick channeled about $560,000 into the law firm during the 1970s to pay Barzel's salary after he stepped down as party chairman. He has contended that he stepped down in 1973 because of policy diferences with the hierarchy after loss of an election six months earlier to the Social Democrats.

Last week, Kohl said the charges against Barzel were "misleading and pure slander." But other party members were less sympathetic.

Party officials said that after Barzel quit as party chairman, he continued to receive half his salary for 30 months to ease any financial difficulties. "Nobody wanted to see him become a social welfare case," a Christian Democratic official said. But Barzel apparently never informed the party hierarchy that he had signed on with the Frankfurt law firm for a large salary or that Flick had become an important client, officials said.

Party officials said there was also a consensus among members to limit further damage to the government parties before important state elections are held in 1985.

Kohl, who came to power two years ago promising "a spiritual and moral renewal" in West Germany after 13 years of Social Democratic-led governments, has seen the credibility of his conservative coalition buffeted.

A perception of business manipulation of parties was fortified by Kohl's attempt earlier this year to push legislation offering amnesty from prosecution for companies caught up in 1,800 cases of tax evasion because of party donations.

Rudolf Diehl, one of Flick's senior accountants, has testified to the parliamentary inquiry that the firm passed more than $8 million to parties in the past decade.

Diehl said about $5 million went to the Christian Democrats, $2 million to the Free Democrats or Liberals, and about $1.5 million to the Social Democrats. Most of these payments are considered illegal because they were laundered through charitable organizations so the firm could claim tax exemptions.

Only the antiestablishment Greens, a party that entered the national parliament for the first time after March 1983 elections, has remained untainted by the firm's donations. Otto Schily, a lawyer who is a key leader of the Greens, said today he wants the party now "to turn all attention to the case of Chancellor Kohl."

Kohl has denied that his path to the top of the party was aided by Flick money and contended that he "never heard anything" about payments by Flick to the Frankfurt law office that hired Barzel.

The current edition of Stern magazine reports that between 1973 and 1980, Kohl, as leader of the opposition, received about $220,000 in "unofficial cash payments" intended for party coffers.

Entitled "Just like in a Banana Republic," the article quotes Flick manager Paefgen as noting in 1976 that "I intend to outfit Kohl in the same way as the other gentlemen."