The Reagan administration said yesterday that it opposes creating a special presidential commission to determine if the nation's antitrust laws have impaired the ability of U. S. firms to compete here and abroad.

Although administration officials told a Senate Judiciary Committee hearing that they believe such a study is needed, current budget restraints, as well as the president's vow to cut the size of the government, made it impossible for them to back a Commission on International Application of U. S. Antitrust Laws.

Sherman E. Unger, general counsel for the Commerce Department; William F. Baxter, assistant attorney general for antitrust; and Davis R. Robinson, the State Department's legal adviser, also said that the focus of the commission, as currently envisioned, is far too narrow.

In addition to examining antitrust laws, the commission also should study "a wide variety of other laws at least as exasperating" to businesses trying to operate abroad, including tax laws, export controls, antiboycott rules, and the antibribery law.

The commission, which has won the strong backing of the American Bar Association and many international businesses, has been proposed for some time but has gained increasing support as American businesses complain that U. S. laws keep them from entering joint ventures that would make them more effective competitors abroad. Foreign governments also have complained that the United States has used its antitrust laws illegally and unfairly against some foreign companies operating here. Antitrust officials here repeatedly have denied both charges.

Nonetheless, at the urging of former senator Jacob Javits, the Senate last year approved by voice vote legislation creating the commission. At that time, the Congressional Budget Office estimated the commission would cost $550,000.

Javits returned to the Senate yesterday to push his proposal again.

Yet, Baxter disagreed with it yesterday, saying it would be far better to conduct bilateral negotiations with countries that disagree with U. S. enforcement of the antitrust laws to resolve the differences.

The current bill's sponsor, however, disagreed. Sen. Charles McC. Mathias (R-Md) argued that the talks wouldn't be effective until there was a national consensus--which wouldn't be worked out unless there was a commission.