President Reagan said yesterday that his administration's plans to combine most government trade activities into one Cabinet department would let both "U.S. business and the world know we speak with one voice" in the trade field.

He told a U.S. Chamber of Commerce-sponsored conference on world trade--beamed by closed-circuit television to 42 cities--that trade is too important to continue the present system, which splits policy formation, negotiations and implementation among a large number of government agencies.

"Ten percent of our gross national product is foreign trade. A billion dollars of exports can add 25,000 jobs, play a factor in solving our unemployment problems," Reagan said in answer to a question from a New York executive.

"We have just decided that, with this great importance to business and industry . . . it makes sense to have a single government agency where policy for foreign trade is concentrated," the president said in his first public comment on the trade reorganization plan.

The administration has proposed tacking on the White House's small Office of the U.S. Trade Representative--designated as the major government voice on trade policy and negotiations--to a large segment of the Department of Commerce to form the new trade department.

The reorganization, announced at the White House April 25 by Commerce Secretary Malcolm Baldrige, has caused great controversy within the trade community. It is unclear, moreover, how much support the proposal has in Congress.

In explaining his support for the reorganization, Reagan focused on the dispersion of trade decisions through the government. But he cited as examples two departments whose interest in trade is unlikely to be diminished by any reorganization: Agriculture, which under the administration plan would keep its power over farm exports, and Defense, which is trying to gain greater control over high-technology exports not under the purview of the Commerce Department.

The president's speech contained a laundry list of administration trade objectives, and included attacks on what he called the "quick fix" of protectionism and the "cruel hoax" of labor-supported local-content legislation that would force Japanese automakers to use U.S.-made parts and American labor in cars they sell in this country.

The thrust of his presentation, however, was the importance of exports as an element for lifting the economy further into recovery. Reagan talked about a potential overseas market of $2 trillion ignored by 90 percent of American manufacturers that, if tapped, could add jobs and provide income security for the American people.

He provided the businessmen --who saw Reagan in Washington and Baldrige and U.S. Trade Representative William E. Brock in Paris, where they were attending a meeting of Western industrialized nations in the Organization for Economic Cooperation and Development--the vision of "a new voyage into the future--a future in which commerce will be king, the eagle will soar and America will be the mightiest trading nation on earth."

In Atlanta, Baldrige drew hisses and catcalls in answering a question from a conference on U.S.-Saudi business that listened in on the program. Baldrige said the administration was doing nothing to return Saudia Arabia to the list of countries that have most-favored-nation status.