Mayor Marion Barry has proposed closing a loophole in the District's tax law that exempts doctors, lawyers and other professionals who have incorporated in the District from paying a 9.9 percent corporate income tax.

The measure, which would raise an additional $500,000 a year in revenue, was part of a package of six proposed revisions in the tax laws that Barry submitted this week to the D.C. City Council.

Jeffrey Humber, director of the D.C. Department of Finance, stressed yesterday that the mayor's tax proposal differs from an ill-fated income tax that the District imposed on 4,500 area professionals during the late 1970s.

That tax, aimed primarily at suburban residents who maintained offices in the District but who paid no city income tax, was struck down in February 1980 by the D.C. Court of Appeals. The city was forced to refund about $40 million that already had been collected.

"The proposed tax doesn't include all professionals as was the case under the old professional tax law , but only those that choose to incorporate in the District," Humber said.

"It's also an effort to clean up our administration in the way we handle professional corporations," he added.

Under the proposal, professionals would be allowed to deduct their salaries from their taxable corporate income, up to 30 percent of the total income. There were no figures available as to the number of professionals who would be affected.

Barry also proposed a number of technical amendments to existing law that would have no significant impact on District revenues.

The proposals include:

* Revisions to the inheritance and estate tax law that would expedite audits and reviews of tax returns.

* Amendments to the personal property tax law, including a provision to tax construction equipment and other tangible personal property brought into the city temporarily and used in a trade or business.

* Changes in the procedures for appealing the city's preliminary assessments of real property.

* Revisions in the law permitting the government to confiscate and auction property to recover unpaid taxes. CAPTION: Picture, MAYOR MARION BARRY . . . measure could raise $500,000