Despite a tide of domestic and foreign criticism of South Africa's latest refusal to reform apartheid, the Reagan administration yesterday defended its policy of quiet diplomacy as the best route to achieve change.

Speaking a day after President Pieter W. Botha defiantly rejected the idea of equal voting rights for South Africa's black majority, Chester A. Crocker, assistant secretary of state for African affairs, insisted the controversial policy of "constructive engagement" -- not punitive U.S. actions -- would be more successful in forcing reform.

"As builders, we must stay engaged," Crocker said to the Commonwealth Club in San Francisco. "Being involved through our presence and our programs means having tools of influence. It does not mean being seduced by a status quo that is overwhelmingly repellent to Americans."

His comments were described by administration officials as an elaboration on the reaction of White House national security affairs adviser Robert C. McFarlane Thursday, a reaction that was carefully neutral. Crocker's remarks followed what the officials called "a night of study and reflection" on the Botha speech and the general reaction to it, administration officials said.

Like McFarlane, Crocker did not criticize Botha, instead stressing that the administration is interested less in what Botha said than in what he does to follow up on his promises of negotiation with leaders of the black African communities.

Crocker characterized Botha's speech as one "written in the code language of a foreign culture within a polarized society," making it difficult to quickly or easily interpret. That statement echoed administration arguments that South African domestic political pressures forced Botha to avoid giving the impression that he was knuckling under to foreign pressures. In that view, judgment should be reserved until Botha has had a chance to demonstrate his intentions through specific actions.

However, some officials also acknowledged that the administration, based on a meeting between McFarlane, Crocker and South African Foreign Minister Roelof F. (Pik) Botha in Vienna last week, had expected the South African president to spell out his ideas in more detail.

Crocker hinted at the administration's disappointment when he said yesterday that the United States had been "informed in general terms" that Botha's speech would "recognize that the process of negotiation had to be launched." But, Crocker added, while Botha did talk of negotiation, his language "does not constitute change and must be followed up."

"We will be looking for concrete results," he said.

Crocker urged the South African government to "get on with it and not get hung up on questions of face and procedure . . . . There is tinder in the townships of South Africa . . . .

"We must get beyond the preliminary stage of bargaining over ground rules -- the shape of the table and who will sit there. The task is to end injustice and racial discrimination," he said.

Crocker, widely regarded as the author of the "constructive engagement" policy, stressed repeatedly that the administration intends to stick with it despite criticism from American civil rights groups and the increasing likelihood that Congress next month will adopt legislation mandating economic sanctions against South Africa.

Botha's speech was greeted in South Africa by almost universal rejection from black leaders and dismay from many white South African business leaders, fearful of the effects of continued violence. The criticism also was echoed here by Democratic and Republican members of Congress.

Asserting that the policy debate is about "what works and what doesn't work," Crocker said that "constructive engagement" had been falsely characterized as unquestioning friendship for the white minority government in Pretoria. Instead, he insisted, it is a tool through which the United States can achieve some degree of influence in a country that is not susceptible to outside pressures.

"It won't be effective to walk away and sever our contacts," he said. "It won't be useful."

He rejected sanctions as a counterproductive tactic that would only hurt blacks and make the chances for accommodation between opposing racial factions more difficult to achieve.

"For us carelessly to throw matches into an already explosive and volatile situation would be a betrayal of such men of peace as Bishop Desmond Tutu," he said. "To turn our backs would be an act of despair."

He cited Tutu even though the Nobel Prize-winning Anglican bishop of Johannesburg said at a news conference yesterday that Botha's speech had convinced him that "the chances of peaceful change are virtually nil."

While Crocker referred repeatedly to the need for democratization and power-sharing with blacks, he declined to specify whether that should be done on a "one-man, one-vote" basis, an idea that Botha rejected in his speech.