Prime Mnister Robert Mugabe said today that the Reagan administration is faced with choosing between friendship with black Africa or South Africa.

The Zimbabwean leader balanced his carefully phrased remarks with praise for the administration for its "very positive" approach in pledging $225 million in aid to the country over the next three years at an international donors conference that ended yesterday.

"Our hope is that the Reagan administration does not move closer to South Africa," Mugabe told a press conference. If it did, he added, "the U.S. administration would naturally have decided to make a choice between us and South Africa, which choice we shall regret but we will continue to work as effectively as possible to make the American administration see things the way we see them."

While he urged the United States not to choose South Africa, Mugabe's own attitude toward his southern neighbor is complicated by Zimbabwe's dependence on South Africa that intensified during 15 years of international sanctions under the former white minority government.

Mugabe said Zimbabwe supports the African move to impose economic sanctions against South Africa over its refusal to grant Namibian independence, "but we cannot involve ourselves in sanctions" because of economic links to the south.

South Africa increased economic pressure or Zimbabwe this week by serving one year's notice that it intends to end the preferentia trade agreement between the two nations.

Mugabe said the South African move "was to be expected. The attitude of South Africa toward us has not been that favorable."

The biggest problem for Zimbabwe's foreign relations is neither East nor West, but rather South Africa. Any American move in the direction of favoring the Pretoria regime could polarize the region and force countries like Zimbabwe to choose between East and West.

A Western analyst here commenting on Africa's antagonism toward white-minority-ruled South Africa noted this week that Western economic aid, although badly needed, would play a secondary role to the issue of South Africa. "There is no way U.S. aid could ever compensate for a wrong position on South Africa on the racial issue, he said."

"It would lose the blacks and knock U.S. aid out of business" in the area, he said. Such a development, he added, would mean the United States could not have a negotiating role in seeking a peaceful alternative to racial conflict in southern Africa.

Just as the Reagan administration seems to be dividing the world between the friends and enemies of the Soviet Union, black Africa regards a nation's attitude toward South Africa as a test of friendship.