John B. Anderson's money-short presidential campaign received a major shot in the arm yesterday when the Federal Election Commission said he is eligible for public campaign financing.

The commission ruled 5 to 1 that Anderson's independent pesidential candidacy is the equivalent of a third-party effort and thus meets the qualifications for federal subsidy.

Anderson won't automatically receive the retroactive funds. Under law, he first must win at least 5 first must win at least 5 percent of the popular vote Nov. 4.

The subsidy, to be paid after election day, would be based on Anderson's portion of the vote. If the Illinois congressman receives 5 percent of the vote, he would be eligible for $3 million, he said yesterday. If his total reached 15 percent, he would get $10.5 million.

Under public election financing law, enacted to reduce the influence of big contributors, the nominees of the Democratic and Republican parties automatically receive $29.4 million. They then are prohibited from accepting donations from private contributors.

To be eligible for $29.4 million, Anderson would need to get 35 percent of the vote. He is allowed, however, to continue to seek private contributions.

The practical meaning of the FEC ruling, Anderson told a news conference, "is to make us eligible to borrow money" on the anticipated subsidy.

"I am very pleased," he said. "I think it will be very significant for my campaign."

The FEC decision was a vote of approval of an advisory opinion, drafted by staff lawyers, that concluded Anderson should "not be excluded from receiving post-election public funds as a candidate of a new party."

The opinion said that although Anderson is running as an independent, his campaign is the "functional equivalent" of a third-party effort. Third-party candidates are eligible for public subsidies under the law.

The opinion noted Anderson has a paid staff of 259 and is on the ballots of 26 states. To meet legal requirements in several states, it added, the Illinois Republican has formed new political parties.

In 1976, the commission rejected a similar appeal from Eugene McCarthy. That 3-to-3 vote divided along party lines, with three Democrats on the panel voting against and three Republicans voting for McCarthy.

Several commissioner yesterday complained about the haste and pressure-packed atmosphere in which the Anderson matter was brought up. For example, Commissioner Frank Reiche a Republican, said Anderson's campaign was "doomed to failure" unless the FEC ruled favorably.

The only commissioner to vote against Anderson was John McGarry, a controversial appointee to the body and long-time friend of House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass) Mcgarry said he favored federal subsidies from Anderson, but he accusedd the commissioner of "ducking the real issue" by declaring his effort a third-party one rather than ruling that independents should be eligible for subsidies.

The ruling came at a time when Anderson's campaign is in severe financial trouble. According the latest financial report, his campaign had raised $5.5 million up to Aug.1, was $476,000 in debt and had $132,000 on hand.

It was the 11th straight legal victory Anderson attorney Mitchell Rogovin. Rogovin originally had challenged the FEC in federal court. A three-judge panel heard oral arguments in the case last week. Rogovin said yesterday he will consider filing a "suggestion of muteness" with the Federal court after he receives the formal FEC ruling.