The Reagan administration denied yesterday that it "has decided to tilt in favor of South Africa's government or its white population," but it also said it will not seek to undermine that country's government or take sides in the conflicts between its blacks and whites.

That point was stressed by Chester A. Crocker, assistant secretary of state for African affairs, in testimony before the House subcommittee on Africa. The administration's policy, he asserted, is to act as a neutral broker able to mediate solutions to southern Africa's racial strife and protect Western strategic and economic interests in the region.

"In South Africa, it is not our task to choose between black and white, but rather to foster conditions in which all South Africans can more fully share and participate in the economy and political process. We seek through our policies neither to destabilize South Africa nor align ourselves with apartheid policies that are repugnant to us," Crocker said.

He added: "To those who would say this administration has decided to tilt in favor of South Africa's government or its white population, we simply reject the charge. If there is a tilt in our policies, it is toward developing greater influence and credibility as a regional partner . . . . "

Crocker's testimony was basically a restatement of an Aug. 29 Honolulu speech in which he attempted to explain the administration's rationale for its policy toward South Africa. That policy has come under increasing fire from the countries of black Africa and from American blacks and liberal whites on grounds that it is encouraging Pretoria to continue its white supremacist rule and carry out military actions against hostile neighbors.

The centerpiece of administration policy involves an attempt to induce South Africa to permit independence for Namibia, a largely black neighboring territory that has been controlled by Pretoria since the end of World War I. In exchange, documents leaked from the State Department have made clear, the administration has offered to help South Africa break out of the political isolation imposed on it by the international community.

Although he didn't put it in such stark terms, Crocker insisted yesterday that this pragmatic approach offers the only realistic hope of resolving the Namibia problem and other regional conflicts such as the civil war in Angola, where Cuban troops and Soviet advisers are helping the Marxist government fight insurgent forces.

Although he refused to give specifics, Crocker insisted that the U.S. initiatives give grounds for "cautious optimism" for "early progress" on a Namibia solution.

The questioning from liberal members of the subcommittee made clear, though, that Crocker had failed to ease their concerns.