The Reagan administration, having dropped the government's 13-year-old antitrust case against International Business Machines Corp., now appears ready to help the computer giant fight off a similar antitrust suit brought by the Common Market.

Administration officials have received permission to attend a private hearing in Brussels this week at which IBM will try to persuade the European Economic Community to drop its antitrust complaint.

EEC procedures will not permit government officials to argue on IBM's behalf at the hearing. However, the State Department has asked EEC officials for an opportunity to "consult" sometime after the hearing and probably will explain why they believe the case should be dropped.

Officials from the Department of Commerce, and perhaps the Department of Justice, are expected to make the case for the administration, arguing among other things that the suit threatens international trade and the ability of American computer firms to compete both at home and abroad.

The EEC suit stems from an investigation begun more than seven years ago. An official complaint was sent to the computer giant in December, 1980. But because EEC procedures require the complaint and subsequent filings and hearings to be kept private, the exact charges have not been made public.

Nonetheless, lawyers familiar with the case indicate that many of the charges are similar to those brought against IBM by the U.S. government and several of IBM's competitors in numerous antitrust suits in this country. IBM won or settled the private suits, while the Reagan administration dismissed the government's suit on Jan. 8 on grounds that it had no merit.

The EEC suit charges IBM with misusing its dominant position in the computer market to keep out competition, lawyers say. Among other things, the suit alleges that IBM did not disclose all details of the new machines it planned to introduce until they were on the market, thus preventing competing firms from making components compatible with these machines until after IBM had captured a large share of the market.

The EEC case "was tried once and lost in our courts," says Commerce Department general counsel Sherman E. Unger, who will be the U.S. government's observer in the EEC hearing.

Some of the proposed sanctions the EEC wants to impose on IBM "could change the way of international trade of technology," Unger added.

Although it is unclear what redress the EEC is seeking, some lawyers familiar with the case say the European community is considering ordering IBM to change some of its marketing practices--for example, ordering it to disclose the technical specifications of new equipment before it is sold.

"This could give all the secrets to somebody else before IBM could get into the marketplace," complained Unger.

Unger and Justice Department officials stress that even though they have asked for consultation rights, the administration has not yet decided whether it will make its views known to the EEC. "The only definite thing that we've decided" is that the U.S. representatives should attend the hearing, they said.