The Reagan administration yesterday announced its plan for creating a new Department of International Trade and Industry designed to help American businesses sell their products overseas and to defend them from unfair trading practices of other countries.

President Reagan, in a statement read at the White House by Presidential Counselor Edwin Meese III, urged quick congressional passage of the trade proposal, which has undergone substantial revision since it was first revealed five weeks ago.

The president said trade has become too important to the U.S economy to allow the present split--with policy set by the Office of the U.S. Trade Representative (USTR) and its implementation under the control of the Commerce Department--to continue.

Under the new proposal, the USTR's role would be enhanced, with its 150 specialists acting in a policymaking role for the head of the new cabinet department, while far less of the present Department of Commerce than originally envisaged would be included. The Commerce Department itself would be abolished in the proposed reorganization.

Meese declined to say who would head the new department, though Commerce Secretary Malcolm Baldrige is reported to be the leading candidate because of his success in winning presidential approval for the reorganization. But U.S. Trade Representative William E. Brock Jr. remains a candidate.

Baldrige, who has said he would resign rather than get involved in a turf battle with Brock, denied persistent reports within the White House that he got presidential authority for the reorganization by reminding Reagan of a promise to give Commerce authority over trade.

"The president had the idea for a long, long time," he said.

The changes appeared aimed at easing opposition to the reorganization both within the administration and among key congressmen and business groups.

Brock confirmed early unhappiness with the makeup of the proposed Department of Trade, but said he is fully satisfied with the present plan. "We had expressed a modest amount of concern early on with the proposal . . . to simply merge USTR into the Commerce Department," he said. "This is not what is being proposed today."

The current proposal, Brock said, is a "very different department, a very new department" that retains all the functions of the USTR as "a clear policy arm of the secretary in the secretary's office."

It is unclear whether this change will be enough to win over the legion of USTR alumni who have led the charge against the White House proposal.

The current White House plan, however, cuts in half Commerce's role in the new department from the working model described by Baldrige a month ago. At that time, he said the new department would contain more than half of the Commerce Department's 35,000 employes, with the National Oceanic and Atmospheric Administration (NOAA)--which takes up 40 percent of Commerce's manpower and 60 percent of its budget--being spun off.

Under the current plan, the new Trade Department would consist of about 7,500 employes from Commerce. NOAA would become an independent agency; the National Bureau of Standards would become part of the National Science Foundation; the Minority Business Development Agency would be transferred to the Small Business Administration, and the Economic Development Administration would be shifted to the Department of Housing and Urban Development. A new home still has to be found for the Census Bureau.

The cutback in Commerce's role appeared to have won the support of Sen. William Roth (R-Del.), who favors trade reorganization but believes Commerce is "a grab-bag of unrelated functions" unequal to running U.S. trade policy. Roth, chairman of the Senate Governmental Operations Committee, is sponsoring a trade reorganization bill that now will encompass the administration's proposals.