The Export-Import Bank, announcing the first use of the Reagan administration's new program to combat subsidized export financing, said yesterday it plans to offer grants to help American companies win sales worth more than $250 million.

Ex-Im Chairman William H. Draper III said the grants, amounting to at least 25 percent of the sales price, will be used to combat "predatory" credit terms offered by foreign competitors in six deals. Although bank officials would not name the countries, sources said France was involved in every deal, sometimes with another nation.

Last month, President Reagan proposed a $300 million "war chest" to fight mixed-credit financing offered by other countries. That is awaiting congressional action, but the Ex-Im Bank already has funds for mixed-credit financing that the administration until now has declined to use.

The administration argues that mixed credits are bad, but decided to use them as a weapon to force other countries to abandon them.

The Ex-Im Bank terms will mix grants -- generally offered to Third World nations for development or humanitarian purposes -- with commercial-rate loans to form a cut-rate package for the buyers. The grant portion of the package would take about $75 million from the presidential "war chest."

The Reagan administration consistently has opposed mixed credits as an unwarranted use of scarce foreign aid funds, but changed its position last month when it formulated a more aggressive trade strategy to fend off protectionist pressures in Congress.

The Ex-Im Bank action was applauded by congressmen, many of whom have been pushing the administration to change its policy and use mixed credits.

Rep. Don Bonker (D-Wash.), chairman of the House Foreign Affairs international economics subcommittee that has been holding hearings this week on the administration war chest, called the Ex-Im decision "good news."

"I hope it is the beginning of an aggressive commitment by the administration to combat predatory financing and to enable our exporters to compete on an equal footing," he added.

Draper, echoing Reagan's rationale for agreeing to support the credits, said, "Our proposed actions are targeted against countries that are hampering the progress of international negotiations to eliminate these predatory practices."

Administration trade officials consistently have singled out France, aided by Italy and Belgium, as the biggest roadblock to negotiations among industrialized nations to place bars on the use of mixed credits. Earlier negotiations set limits on subsidized credit terms that can be offered to win sales, but left open the mixed-credit loophole.

Japan has been cited as another major user of mixed credits to win contracts for its companies, even though it is not actively blocking the negotiations, being conducted through the Organization for Economic Cooperation and Development, U.S. officials said.

The Ex-Im Bank said the six U.S. deals targeted for mixed credits involve sales in the transportation, computer and power fields. "All the transactions involve intense international competition for sophisticated equipment and services which should be financed on commercial terms. None involves a project for which foreign aid should be used," the bank said.

Draper declined to name the U.S. companies involved in the bidding, but said their offers all are considered competitive in commercial areas of price, technology and performance. But their bids suffer because of government-aided mixed credits offered by companies from other countries, he said.