In a move that may add to the economic difficulties of its black-ruled neighbor, South Africa has decided not to renew the work permits of 20,000 Zimbabweans employed in South Africa when the permits expire during the next 12 to 18 months.

A government spokesman said the South African decision complied with Zimbabwe's announced desire to keep its nationals home. The action nevertheless will probably be regarded in Salisbury as one more turn of the economic screw, and may intensify the recent souring of relations between the two countries.

In many small ways the South African government has indicated an unwillinghess to estend more than the minimum of aid to Zimbabwe. In contrast, it gave generous economic backing to the former white minority government in Salisbury.

South Africa has refused to continue leasing 21 diesel locomotives to the Zimbabwean railroads, saying it needs them at home. It also refused to renew the lease of an airplane to Air Zimbabwe. In contrast, it gave generous economic backing to the former white minority government in Salisbury.

South Africa has refused ton continue leasing 21 diesel locomotives to the Zimbabwean railroads, saying it needs them at home. It also refused to revew the lease of an airplane to Air Zimbabwe, putting the plane up for sale instead.

In May, South Africa announced that its preferential trade agreement with Zimbabwe would not be revewed when it runs out next year.

South African Prime Minister Pieter W. Botha said of this action that he did not think cooperating amicably with neighboring black states meant that South Africa "should play Father Christmas" to them.

In explaining South Africa's decision not to revew the work permits, Johan Oosthuizen, a spokesman for the Ministry of Cooperation and Development, which deals with black labor, said that Zimbabwean Information Minister Nathan Shamuyaria announced in February that his government would not longer issue travel documents to persons who wanted to work in South Africa.

Reports at the time, however, indicated only that Shamuyarira discouraged Zimbabweans from seeking work in South Africa and voiced his government's disapproval of the practice.

In addition, a spokesman for the Black Sash, a South African organization that helps blacks through the maze of regulations that govern their lives, said no white employers had complained of Zimbabwean interference with travel of workers to South Africa.

At any rate. South Africa labor bureaus on July 13 were ordered not to renew the work contracts of Zimbabweans since they would not have the required travel documents, Oosthuizen said.

"It was not a decision by our government," Oosthuizen said. "You cannot have employes in your country without the consent of their government."

Zimbabwe has had a hand in the loosening of economic and labor relations between the two countries. Like its black-ruled neighbors, it has declared its intention to reduce its economic dependence on South Africa. One way the black states have sought to do this is by decreasing the number of their citizens who work in South Africa.

In February, Zimbabwe refused to revew the license of Wenela, the South African mine labor recruiting agency, thus preventing it from seeking mine workers in Zimbabwe.