The South African government is buoyant with expectations about improved relations with the United States and, on the eve of a visit by Assistant Secretary of State-designate Chester Crocker, eager to talk with him about one of this country's major concerns: Cubans in Angola.

The hopes for closer ties flow from a series of encouraging signals from Washington, including:

Official comments on the need to halt Soviet expansion and international terrorism, disclosure that Prime Minister Pieter W. Botha might be invited to Washington, suggestions that the United States might give assistance to anti-Marxist guerrillas in Angola, willingness of U.S. officials to meet recently with South African-backed political parties from the territory of Namibia, and President Reagan's remarks about South Africa in a recent interview.

"Suddenly things don't look so bad for South Africa," commented the Afrikaans newspaper Beeld. "It is as if a thin line of light has appeared in the east which heralds the dawn of a new day."

The most positive reinforcement came last weekend with Reagan's statement, in an interview with The Washington Post, that the United States favors altering a Western-designed U.N. plan for independence in the Pretoria-run territory of Namibia (also known as South-West Africa) to draw up a constitution before elections there. South African officials welcomed the proposal, one they have promoted for months.

At the core of South African expectations is hope that the Reagan administration will view the Soviet and Cuban military presence, rather than South Africa's racial policies and military strength, as the principal security threat in this region.

South Africa's preoccupation with the Soviet and Cuban presence stems in part from its own anti-Marxist beliefs and a conviction that the Soviets are after its mineral riches. But it also reflects an effort by Pretoria to build up unchallenged military and economic strength in southern Africa to discourage neighboring states from assisting anti-South African guerrillas.

For South Africans the litmus test of American resolve to stop Soviet moves in this part of the world will be in Angola. South Africans would like to see the United States end the congressionally imposed ban on U.S. aid to Angolan factions, then provide open assistance to the anti-Marxist UNITA guerrilla movement fighting the Angolan government.

In 1975 South Africa intervened in the Angolan civil war on the side of UNITA and has retained close ties with it since then.

As for Namibia, Reagan's proposals to draw up a constitution before elections will give the South Africans the two things they want there; time and more international recognition for political parties that will oppose the guerrilla movement SWAPO (South-West Africa People's Organization) in elections.

Apart from Angola, Pretoria's most immediate concern is a decision by Washington on whether it will supply enriched uranium for two nuclear power plants under construction. Washington has held up the fuel because of South Africa's refusal to agree to international inspection of its entire nuclear program amid suspicions it is working on nuclear weapons. The issue is becoming crucial because the first of the two plants is due to begin operations next year.

South Africa would also like to see U.S. Navy ships once again at its ports. There are also some hopes that the United States will unilaterally lift a mandatory arms embargo imposed by the United Nations in October 1977. Although the government boasts that the boycott has not hurt it and has even spurred its own arms industry, it would like to buy some sophisticated weapons and fighter planes.

The South Africans also want to be able to buy a number of items the United States has unilaterally embargoed, because of their possible military or police use, such as computers, spare parts, police equipment and planes for reconnaissance and sea rescue.

But at the minimum, South Africans are hoping Reagan will give them "breathing space" in the form of a moratorium on public pressures for faster changes in their domestic racial policies and elimination of linkage between such changes and bilateral relations.

"To a certain extent, this is already happening," said international affairs analyst John Barratt. "[Former president Jimmy] Carter said you cannot expect warmer relations while your internal policies remain the way they are. But under Reagan, it seems to be business as usual so far."

Crocker has argued that high-profile rhetoric and criticism is not effective in moving Pretoria. He is known to believe also that while the United States is pressing Pretoria for cooperation on a settlement in Namibia, it ought not to be pressuring it for changes at home.

Aware, however, that most key policy decisions affecting them have yet to be made, some South African officials are cautioning their colleagues and the public about reading too much into the preliminary signs out of Washington.

Foreign Minister Roelof (Pik) Botha recently warned a meeting of party supporters against a "spirit of euphoria" since Reagan's election. "There are people in South Africa who have grabbed at the election of Mr. Reagan as a drowning person grabs at a piece of driftwood," Botha said, "but in the weeks and months ahead we and the U.S. will not always agree."

It was an action by the South Africans themselves, probably born out of unrealistically high expectations about the new administration, that has no doubt contributed to a lowering of those expectations.

Despite a long-established ban on visits to the United States by South African military officials, five high-ranking officers, including the head of military intelligence, traveled to Washington in March. Their visa applications to the U.S. Embassy here did not contain their military ranks.

The State Department struck a low-key response to their "inadvertent" visit. But public disclosure of their presence and of the fact that some of them met with the U.S. ambassador to the United Nations, Jeane J. Kirkpatrick, caused the administration embarrassment. It also angered some who saw it as an example of South African attempts to use backdoor diplomacy to influence Washington.

"They tried to sneak past us," said one official. "That's not nice."