The Reagan administration, in what a key State Department offical yesterday called "our fullest statement to date on the subject," has said it will not "choose between black and white" in its dealings with racially segregated South Africa.

The administration's southern African policy, largely based on the strategic and economic importance of the region to the West, was outlined over the weekend in a speech by Chester A. Crocker, assistant secretary of state for African affairs.

Crocker told an American Legion convention in Honolulu Saturday that South Africa's racial separation policies "are abhorrent to our own multiracial democracy."

But he said any move by the Reagan administration to choose between the white minority South African government and its black majority populace, or nearby black African-ruled countries, might destabilize the area and undermine any chances for peaceful solutions to long-running conflicts in the sub-Saharan region.

Such a development would harm American interests, Crocker said.

The administration's policy statement came in the midst of Third World and some western protests over a South African military strike last week into neighboring Angola.

The incursion, which has not yet ended, was the latest of a number of South African military operations in Angola that the government in Pretoria says are necessary to combat guerrilla operations directed against its control of the territory of Namibia (Southwest Africa).

A number of African countries and some western allies, Britain and France among them, have urged the United States to join in protest against the South African raids.

The administration has declined to speak out on the matter, and an administration source familiar with the development of Crocker's speech said yesterday the speech was not timed to address the issue of the raids.

The speech "was in planning for months," the source, who requested anonymity, said. "There was not much opportunity to give it before. It was coincidental that it came at this time."

The official said the speech was meant to "put into context what the administration has been doing for the past six months" in establishing U.S.-African policy.

In his speech, Crocker told the legionnaires, "We cannot and will not permit our hand to be forced to align ourselves with one side or another in these disputes. . . . In South Africa, the region's largest country, it is not our task to choose between black and white. In this rich land of talented and diverse peoples, important western economic, strategic, moral and political interests are at stake."

The United States and other western governments have long tried to work out an agreement between South Africa and neighboring black nations on a volatile political issue: how to grant independence to South African-controlled Namibia.

Namibian independence fighters, or guerrilla forces, depending on who is describing them, have been operating in Angola with the assistance of the Cuban troops there.

Crocker said in his speech that solving the Namibian problem in a way "internationally acceptable" would do much to reduce tensions in the region.

The United States has made progress in ending the Namibian dispute "in extensive consultations with all parties" involved in the matter, Crocker said.

He said the administration is "now working closely with our European and Canadian allies" in an effort "to shape concrete proposals to put before the parties in southern Africa."

Crocker said the sub-Saharan region "contains immense deposits of many strategic minerals which are vital to industrial economies like ours."

But he said "southern Africa's potential economic dynamism becomes a mirage" if the area is allowed to "slide toward regional turmoil" through aggravation of its racial and political disputes.

The speech drew criticism from Donald F. McHenry, former U.S. ambassador to the United Nations under former President Carter, whose African policies apparently are being discarded.

"In a sense, it's sad," McHenry said in an interview yesterday. "We're falling right back into a pattern that we were in before, following a very short-sighted program in pursuit of our own narrow interests."