The Reagan administration yesterday issued its most pessimistic economic forecast since 1981, predicting that growth will slow to a 2.4 percent pace next year because of the October stock market plunge and the Federal Reserve Board's "tightening of monetary policy."

In a statement accompanying the forecast, Beryl W. Sprinkel, chairman of the president's Council of Economic Advisers, implicitly warned the Fed to ease credit soon. The statement, which asserted that "an appropriate monetary policy is especially important," appeared to reflect the concerns of senior administration officials for the need to avoid higher interest rates and recession in the coming election year.

The administration's forecast isn't quite gloomy enough to constitute a prediction of a so-called growth recession, in which the economy expands while unemployment also rises. The administration expects civilian unemployment next year to hover at the current 5.9 percent rate. Thus, although the economy's growth will be "somewhat restrained," 1988 will still be "a good year," Sprinkel said at a news briefing.

The 2.4 percent growth figure is more than 1 percentage point below the administration's most recent forecast, issued in August, which predicted that the gross national product would grow by 3.5 percent, adjusted for inflation, from the fourth quarter of 1987 to the fourth quarter of 1988.

The downward revision closely tracks reassessments of the economy by most private forecasters in the wake of the Oct. 19 stock market plunge. In the past couple of years, the administration -- once the butt of ridicule for its "rosy scenarios" -- has brought its forecasts to levels generally regarded as more realistic. Last December, for example, the administration predicted growth of 3.2 percent in 1987; based on what has happened so far this year, the economy is expected to exceed that figure, perhaps by several tenths of 1 percentage point.

The only year for which the Reagan administration forecast a lower growth rate than 2.4 percent was 1981. Shortly after President Reagan took office, the administration predicted that the economy would stagnate in 1981 because of Carter administration policies, then surge once Reagan policies took effect. The economy did perform anemically in 1981, and then fell into a deep slump.

The administration, like most private forecasters, is expecting consumers next year to slow spending substantially. This slowdown in consumption will be offset by continued strong growth in exports and by a modest pickup in business capital spending, according to the administration.

Behind the administration's current assessment is concern that the Fed has held credit too tight. While refusing to criticize the Fed directly, Sprinkel said that the central bank had drained away most of the money it injected into the banking system after the stock market debacle. He said that for the economy to attain even the modest growth pace predicted by the administration, the Fed must "get {money supply growth} back into their targets." That would imply a relatively rapid pickup in money-supply growth; the money supply declined in November.

Fed officials, who tend to place more importance on interest rates than on money-supply figures, have disputed suggestions that they changed the easier-credit stance they adopted after the market drop.

The fact that the administration is subtly prodding the Fed to keep interest rates from rising runs somewhat counter to its recently stated desire to brake the dollar's descent. Higher U.S. interest rates tend to buoy the dollar by making dollar-denominated investments more attractive.

Some officials of allied governments, upset about the dollar's fall, have urged the United States to raise interest rates. But Sprinkel observed that the statement on international economic policy issued Tuesday evening by the United States and six other industrialized nations doesn't say anything about using interest rates to affect currencies.

The administration's forecast suggests that officials are expecting the economy to be soft enough next year to allow interest rates to fall a bit. The administration predicts that three-month Treasury bill rates will dip to an annual average of 5.3 percent in 1988 from the 5.8 percent average rate of 1987. Ten-year bond rates will drop to 8 percent from 8.4 percent, according to the administration.

Lower interest rates would help shrink the federal deficit, although slower growth would widen the budget gap by causing a drop in tax receipts. In general, the administration's forecast -- which is used in preparing the president's fiscal 1989 budget, scheduled for release in February -- probably won't cause major problems for the White House Budget Office in meeting the deficit targets specified in the Gramm-Rudman-Hollings law, officials said.

One reason for this is that the administration projects relatively strong growth in 1989 and beyond, even though interest rates and inflation -- which usually rise with a booming economy -- are projected to decline. Sprinkel said that the administration's projections for 1989 to 1993 aren't based on a prediction of what will likely happen, but rather on a forecast of what could happen if appropriate economic policies are pursued.