South Africa's steps this week to ease its system of racial segregation resulted from a commitment to move away from apartheid and were not a response to sanctions imposed by President Reagan against Pretoria Monday, ambassador-designate Herbert Beukes said yesterday.

"You cannot influence someone if you antagonize them," Beukes said in a meeting with editors and reporters of The Washington Post.

Instead, he contended, President Pieter W. Botha's pledge Wednesday to restore the citizenship lost by millions of blacks when the white-minority government established nominally independent black "homelands" was the result of decisions made weeks ago.

Similarly, Beukes said, the U.S. sanctions were not a factor in yesterday's recommendation by a South African advisory panel to abolish "pass laws" restricting blacks' movements and controlling their right to live in urban areas.

"The South African government, if you understand the way it works, is obviously not going to decide between three days ago and now that the pass laws or other fundamental features of our system are going to change," he said.

Such changes, Beukes added, are part of a process that has caused the Botha government to decide that apartheid must be replaced by a new system that will give political equality to all South Africans.

"We in South Africa realize that we must move away from the status quo. The status quo will not be maintained and will not be defended," he said. "We may not have moved much away. But I think we have moved so far that you can at least see what we've done."

In that respect, Beukes said, the limited sanctions imposed by Reagan in response to congressional and public pressure are likely to prove counterproductive because they will make white South Africans angry and defensive, while having little practical effect on their lives.

Steps ordered by Reagan include limits on computer sales and loans to the government and a possible future ban on the sale in this country of gold coins called krugerrands.

"If it had been sanctions of an international dimension, something total like a blockade, then it would be a serious threat," Beukes said.

But, he added, "I don't believe a government is going, if it were not willing to do so and had not realized the need to do so of its own accord, to make fundamental departures from a whole ideology . . . on the basis of bank loans not being available to the government or that krugerrands might not be sold here."

Reagan had been pursuing a controversial policy of "constructive engagement" aimed at influencing South Africa through friendship and quiet persuasion. He initially opposed sanctions and finally acted only because Congress was about to enact tougher measures.

A procedural move by Senate Republicans yesterday forced Democrats to halt, at least temporarily, efforts for stronger sanctions. Twice this week, the Senate declined to end a filibuster blocking a vote on the sanctions bill, and yesterday's procedural move effectively foreclosed another vote.

Despite American liberals' criticism of constructive engagement, Beukes said the policy actually helped to further South Africa's movement toward reform.

Specifically, he said, U.S. cooperation and sympathy for Pretoria's position in its disputes with neighboring black African states had "provided a feeling of stability that allowed us to move ahead with reforms which otherwise might have met greater resistance from our white electorate."