The budget agreement between President Reagan and congressional leaders probably won't shrink the federal deficit enough to achieve the target set by the budget law for the next fiscal year, according to administration and congressional officials and private budget analysts.

As a result, these officials and analysts said, the threat of an automatic spending cut will loom once again in the summer of 1988 -- unless the economy performs substantially better than most forecasters expect.

The likelihood of a new budget imbroglio erupting during next year's election campaign underscores the limitations of the recent budget pact. Critics have complained that the accord won't narrow the deficit much below the $148 billion level recorded for fiscal 1987, which ended Sept. 30.

One of the pact's supposed virtues was that it would last for two years. But "it's not big enough to save us from sequestration next year," said Robert Reischauer, a budget expert at the Brookings Institution.

Sequestration is the automatic cuts in military and domestic programs required under the Gramm-Rudman-Hollings law. The cuts go into effect if the White House and Congress fail to achieve the deficit target set by the law.

The legal target for fiscal 1989 is $136 billion, and the law permits the target to be exceeded by $10 billion. But if the economy slows down next year as expected, the deficit for 1989 will be "somewhere in the 160s," Reischauer said.

A projected 1989 deficit in the $160 billion range would trigger the automatic-cut process in August 1988, requiring Reagan and lawmakers to confront anew the choice between saving more or allowing indiscriminate cuts to take place.

Reischauer said his deficit estimates were based on the assumption that the Oct. 19 stock plunge will cause the economy to slow next year, as forecast by the Blue Chip Economic Indicators, a widely used consensus forecast of private economists.

Administration sources said the White House budget office has arrived at similar deficit projections using the Blue Chip forecast, and congressional sources said the Congressional Budget Office has undertaken a similar analysis and arrived at similar conclusions.

Based on economic forecasts prepared before the market crash, both the White House and congressional budget offices predict that the deficit-reduction pact would reduce red ink below the Gramm-Rudman-Hollings targets for 1988 and 1989. But those forecasts are in the process of being updated in the wake of the stock plunge, and are expected to reflect factors that tend to widen the deficit. Chief among these factors is slower economic growth, which causes tax revenue to decline.

The most recent Blue Chip survey, which is compiled by Eggert Economic Enterprises Inc. of Sedona, Ariz., showed that private economists expect the economy to grow at a 2 percent rate in 1988, adjusted for inflation.

The Reagan administration's most recent forecast projected economic growth of 3.5 percent in 1988. Chief White House economist Beryl Sprinkel has said that he estimates the market's woes may knock a percentage point off of growth next year.