As Eastern European business dwindled, the republics of the former Soviet Union shiftedthe majority of their foreign trade to Western nations for the first time last year, according to a report.

The Western share of exports from republics that now make up the Commonwealth of Independent States soared to 57 percent last year from less than 46 percent in 1990. More than 58 percent of imports came from the West compared with 48.5 percent the year before.At the same time, the share of trade with the former East bloc declined nearly to the level of that with the Third World, said the report by PlanEcon Inc., a Washington-based consulting group.

Using new commonwealth reporting methods, the study found that trade with the West declined far less than that with the former East bloc during last year's economic slump.

While total commonwealth exports dropped 33 percent from a year before, exports to the developed nations fell just 16 percent. Commonwealth imports

from the West declined nearly 33 percent, less than the 44 percent loss in imports from all nations.

By comparison, commonwealth exports to the former East bloc plunged 57 percent from a year before as imports fell 62 percent, the report said. Much of the trade shift from East to West can be traced to the effects of hard- currency trade and higher prices for Russian oil.

Germany emerged as the most important foreign trading partner of the commonwealth by far, accounting for 13 percent of the republics' exports and 17.2 percent of their exports last year. Italy ranked second with 6.6 percent of exports and 6.5 percent of imports, followed by Czechoslovakia with a 6.4 percent export share and 5.1 percent of commonwealth imports.

While U.S. total trade with the commonwealth was less than that of Japan, its share of commonwealth imports ranked second behind Germany at 8.3 percent last year. The U.S. share of commonwealth exports was just 1.5 percent, compared with 4.6 percent for Japan. The Japanese share of imports was 5.3 percent.

Poland, France and Hungary all recorded smaller trade shares in a sign that the East bloc breakup has cleared the way for Western competition. The Netherlands also captured a significant 5 percent share of commonwealth exports as a result of Russian oil shipments to Rotterdam.

Advocates of reorientation toward the West are likely to welcome the statistical shift away from traditional Soviet markets. Given the enormous dislocations of transition and declining regional economies, however, it may be hard to argue that the change denotes a trend.

The economic crisis has given rise to a number of "paper" gains that could vanish when the situation sorts itself out.

Russia, for example, more than doubled its foreign trade surplus outside the commonwealth last year. But the increase was certainly not the result of economic strengthening. Russian exports fell 29 percent, less than the 46 percent decline in imports, accounting for the surplus change.

The figures for the 11 member nations of the commonwealth and Georgia are the result of new statistical reporting, which denominates all trade in ''valuta rubles." The new accounting entity, with an exchange rate of 1.74 valuta rubles to the dollar, is said to be a more reliable method of comparing transactions in hard and soft currencies. Russia's foreign trade surplus would be the equivalent to $11.2 billion in the new terms.

Despite economic problems, the Russian trade position appears far stronger than that of other commonwealth republics. Russia's current account including gold sales showed a $7.1 billion surplus last year, with $3.4 billion in hard currency. The remaining commonwealth republics had a $9 billion deficit in their collective total current account, PlanEcon said.

The report also cited commonwealth estimates that its hard-currency debt stood at $80 billion at the end of last year, a higher figure than in recent Western reports. The figure is also said to exclude trade arrears with former East bloc nations as well as current barter and clearing debts with other countries, which could add another $10 billion.

Of some $158 billion in Soviet loans to client states and the Third World, only $20 billion to $30 billion is collectible, PlanEcon said.