A house Banking subcommittee voted yesterday to prohibit the Export-Import Bank from supporting any private business transactions with South Africa unless that country makes "significant progress" toward equality for its black majority.

If the House approves the subcommittee's action, the result would be a potentially devastating blow to South Africa's ability to purchase goods and services from the United States.

Despite existing restrictions on its dealings with South Africa, the Export-Import Bank is an important source of funding for trade between the two countries. It presently has more than $204 million invested in various programs to support South Africa purchases of U.S. exports.

In another action yesterday, the subcommittee voted to exempt China from communist countries unless the president certifies that the transactions are in the national interest.

These action were taken by the subcommittee on international trade as it approved legislation raising the Export-Import Bank's lending authority from $25 billion to $40 billion.

The bank is a government controlled and funded agency whose purpose is to book U.S. foreign trade by providing financing foe overseas sales by American firms. President Carter has asked Congress to increase the bank's lending powers as part of an administration drive to reserve the adverse balance of U.S. trade

However, the administration did not seek the amendments relating to South Africa and China that were tacked onto the bill by the subcommittee. Both touch on sensitive foreign policy considerations, with the South Africa amendment involving the controversial question of whether, and to what extent, U.S. economic power should be used to force another government to make changes in domestic policies.

Present U.S. economic policy toward South Africa has come under increasing fire from human rights activists, leaders of the American black community and other opponents of that country's white-supremeacy policies.

The critics charge that Washington's lenient financial and trade policies in dealing with South Africa have helped to progagate a system where a white minority is able to afford the high cost of repressing the 83 percent of the population that is black or colored.

For example, the State Department pursues a "neutral course" of neither opposing nor encouraging U.S. business investment in South Africa. Similarly, during the past two years, the United States has helped South Africa obtain loans of $463 million from the International Monetary Fund to deal with internal economic problems.

The one notable area of restriction has involved the Export-Import Bank. Since 1964, the bank has been prohibited from making direct loans to South Africa purchasers of U.S. goods.

However, the bank has continued to provide various programs of insurance and other gurarantees for loans obtained from private U.S. banks by South African importers of U.S. exports. It was this area of the bank's activity that the subcommittee moved to close off when it voted, 10 to 5, to adopt the amendment offered by Rep. Paul E. Tsongas (D-Mass.).

Specifically, the amendment bars the banks from supporting any export transactions with South Africa "unless and until the president determines that signficant progress toward majority rule has been made in South Africa and transmits to Congress a statement describing that progress."

House sources said last night that the ultimate fate of the amendment was unclear. They predicted that it will be the subject of a bitter fight when the parent Banking Committee considers the bill, possibly next week. But, they added, if it gets past the committee, the expectation is that it will be approved by the House.

The amendment involving China was offered by Rep. Les AuCoin (D-Ore.) and adopted by a vote of 11 to 4. As AuCoin acknowledged, its purpose - encouraging improved commercial relations with China - is largely symbolic, since the Chinese have not sought any bank credits from the United States.

Under existing law, the bank cannot make loans to communist countries unless the president grants an exception. Such presidential exemptions have been made in the past for Poland, Romania andYugoslavia.