South African official said today that his government plans to open negotiations this month with Zimbabwe on renewal of a preferential trade agreement that it previously had given notice would expire March 31.

The accord, negotiated long before African nationalist Prime Minister Robert Mugabe brought black-majority rule to Zimbabwe, exempts exports of that country from tariffs and quotas here. Trade figures between the countries are difficult to obtain, but Zimbabwean officials make clear that the agreement is important to their troubled economy.

South Africa is by far Zimbabwe's biggest trading partner but has been restricting commerce since Mugabe's election and the cutoff of diplomatic ties.

South Africa's hard-line policy toward the Mugabe government has been a source of concern to Washington, which regards a stable Zimbabwe as essential for the stability of the whole southern African region.

Diplomatic sources here and in Zimbabwe say South Africa's attitude began to ease shortly after Assistant Secretary of State Chester Crocker visited South Africa and Zimbabwe two months ago as head of a five-nation Western contact group that is trying to negotiate independence for Namibia. These sources said they believe the United States pressed South Africa to reconsider letting the accord lapse.

In quick succession after Mugabe came to power, South Africa had cut off previously agreed loans, withdrawn 25 diesel locomotives and 150 mechanics that had been on loan to Zimbabwe--almost crippling its hard-pressed railroad system--and slowed the turnaround times of landlocked Zimbabwe's rail traffic through South Africa.

South Africa also began repatriating black Zimbabweans working in this country and gave notice on intent to end the trade agreement.

Antigovernment rebels in neighboring Mozambique, whom Mugabe says South Africa is supporting, sabotaged Zimbabwe's only alternative railroads to the sea at the Mozambican ports of Beira and Maputo.

Mugabe accused South Africa of trying to destabilize his country.

But two weeks after Crockers's visit, two Zimbabwean officials visited South Africa to negotiate the resupply of the 25 locomotives. The turnaround times of Zimbabwean traffic through South Africa speeded up and South Africa agreed to an easing of repayment terms on the three loans made to what was then Rhodesia.

Today, the director general of South Africa's Department of Industries, Commerce and Tourism, Tjaart du Plessis, acknowledged in response to a questioner in Pretoria that he expects a meeting with Zimbabwean officials on the trade agreement to take place this month. There has been no official announcement.

On Tuesday, South Africa also changed its mind--following pressure from the United States and other Western countries--on prosecution of 45 mercenaries who hijacked an Air India plane to South Africa when their coup attempt in the Seychelles Islands failed last November. Until Tuesday, South Africa had indicated it would let 40 of the mercenaries go free, but they are now being indicted.

South Africa's earlier tough actions against Zimbabwe appear to have had three objectives: to impress upon its black neighbors their economic dependence as a warning against aid to guerrillas of the outlawed African National Congress here who are trying to overthrow white-minority rule; to put a price tag on the antiapartheid rhetoric of its black neighbors, and to force Zimbabwe into reopening diplomatic relations.

After independence, Mugabe announced that he would continue normal trade relations but not diplomatic relations with South Africa.

Ever since, South Africa has responded to Zimbabwe's complaints about the economic squeeze by insisting on minister-to-minister representations. Zimbabwe refused, accusing South Africa of using its economic stranglehold to try to coerce political relations.

Finding a way around this deadlock was not the only problem Western mediators had to overcome. Another related to Zimbabwe's admission, after independence, to the Lome Agreement between the European Community and former colonial territories.

This stipulated that all existing trade agreements, including the one with South Africa, could stand, but no new agreements could be entered into which contained preferences not granted to EC countries.

The South African agreement contains many such preferences. Therefore, if it ended on the notice date, March 31, it could not be revived afterward.

Urgent pressures have been required to persuade South Africa to rescind its notice, according to diplomats here, so that the agreement in fact remains continuous, although a number of clauses are to be renegotiated with the Zimbabweans.