South African Foreign Minister Roelof (Pik) Botha's meeting scheduled for Thursday in Washington with Secretary of State Alexander M. Haig Jr. is seen here as offering a test of the Reagan administration's policy of "constructive engagement" with this last white-ruled nation in Africa.

The guiding hand of this policy is Assistant Secretary of State-designate Chester Crocker, who recently told the Voice of America, "We do intend to build a new constructive relationship with South Africa which we think will produce more results than some of the efforts made in the past."

Principal issues in the Haig-Botha talks are likely to be South Africa's response to U.S. proposals for ending the bush war in Namibia (South-West Africa) and leading that South Africa-run territory to independence, as well as Pretoria'a attempts at nuclear development.

The Reagan administration so far has vetoed a U.N. Security Council resolution that would have imposed economic sanctions on South Africa because of its policy on Namibia. In addition, U.S. officials have said the Pretoria-backed parties there must have a more visible role in any settlement.

They have also stated that South Africa's concern about the presence of Cuban troops in neighboring Angola is a legitimate one that ought to be addressed, possibly even before a settlement is implemented in Namibia.

Finally, by sending the U.N. plan for a Namibian settlement back to the drawing board, Washington has given South Africa what is most wanted, more time.

The Reagan administration's sympathetic stance has led South Africans to pin much hope on the Haig-Botha talks. The Afrikaans newspaper Beeld, which is connected to the government, commented that Botha's visit "could still become one of the most important discussions in which South Africa has taken part since the Second World War."

South Africans seem now to be hoping for some concrete gesture such as an increase in the personnel in the defense attache sections of the U.S. and the South African embassies. There has been only one attache in each embassy since mutual explusions of officers in 1979. South African officials say they would also like to see U.S. Navy ships allowed to call at South African ports again.

In addition, the South Africans would like to see U.S. measures to force the Cubans to withdraw from Angola. Their presence there, Botha said Monday, was prolonging the civil war in Angola and their withdrawal was "necessary in the interests of southern Africa."

But the topic of most immediate concern to South Africa is probably the one on which it will get the least satisfaction -- deadlock over U.S. supply of low-inriched uranium to fuel two nuclear power plants under construction here.

Under a 1974 Department of Energy enrichment contract, the United States was to enrich raw uranium supplied by South Africa. But a complication arose in 1978 when Congress prohibited the export of enriched uranium to any country that did not sign the Nuclear Nonproliferation Treaty.