NEW YORK (AP) _ President Reagan's proposed anti-apartheid sanctions against South Africa are unlikely to have much effect on U.S. companies doing business with that country, nor are they likely to affect Krugerrand sales here much, financial observers said Monday.

The main reason, they said, is that the growing world sentiment against South Africa's system of racial separation already had worked to restrict trade in the areas that might be affected by the proposed sanctions.

One example was the move announced by Reagan on Monday to ban imports of Krugerrands - the one-ounce gold South African coins - subject to consultations with America's major trading partners.

The impact of that announcement already had been ''discounted'' in the marketplace, as could be seen in the slumping sales of Krugerrands, said Charles R. Stahl, editor of the New York-based Green's Commodity Market Comments.

In his message, the president also said he was prohibiting bank loans to the South African government, except for those going to programs helping blacks; banning all computer exports to South African law enforcement agencies; and halting all exports of nuclear technology until Pretoria agreed to meet the terms of international non-proliferation agreements.

Analysts said those measures would not affect American companies much because of the limited amount of business involved.

A senior administration official, briefing reporters at the White House on condition he not be identified, said banning imports of Krugerrands was the only penalty that marked a genuine change in existing policy.

Currency traders noted the proposed ban on Krugerrands would affect only imports of new coins, not sales of coins already in the United States.

''There are so many millions of these coins going around, it is hard to believe there is not going to be a market,'' said Alan Posnick, vice president at Manfra, Tordella & Brookes Inc., a currency trader.

Even so, the popularity of the South African currency has been plunging steadily this year.

At Posnick's company, Krugerrands outsold Canada's Maple Leaf gold coin by about a 9-to-1 ratio three years ago. By early summer, the ratio had reversed - to 7-to-3 in favor of the Maple Leaf, and in the last few weeks it has stood at about 9-to-1 in the Canadian coin's favor, Posnick said.

Reagan's announcement should cause little change in that ratio, he said.

Before the recent further slowdown, Stahl had estimated that Krugerrand sales dropped from 200,000-250,000 ounces a month last year to 44,000-60,000 ounces a month.

Posnick said the Krugerrand has been trading at a 1 percent discount to a 3 percent premium to the price of gold.

''But a $10 bill is worth $10. A Krugerrand is an ounce of gold,'' he said. Thus, the value of the gold alone protects it from losing much value, except for a discount that might cover meltdown fees.

On Monday, when gold was trading at $321 an ounce, Posnick said his company would buy Krugerrands at $320 and would sell them at $325.70. One-ounce gold Canadian Maple Leaf coins were selling at $330.

On the other areas of Reagan's proposal, observers suggested there was little likelihood that American companies would suffer from the sanctions.

Livia Asher, a banking analyst at the investment firm First Boston Corp., said the loan restrictions should not hurt U.S. banks, because they already had been ''pulling away from'' giving South African loans because of the public outcry.

Stanley Rubin, an analyst at the Merrill Lynch investment firm, said the proposed sanctions would not have much impact on the major exporters of nuclear technology.

''For General Electric and Westinghouse, South Africa represents considerably less than 1 percent of their sales and assets, and probably less than that in terms of profits,'' he said.

On the computer restriction, Frank Halpern, an analyst at the investment firm of Martin Simpson & Co., said, ''The South Africa market is relatively small to begin with, and restrictions on computers to law enforcement agencies there would amount to a virtual half-drop in the bucket.''