In a drastic turnabout from the explosive growth of the 1960s, the population of Prince George's County dropped by more than 22,000 between 1970 and 1980, according to preliminary results from the 1980 federal census.

The 3.4 percent decline brought the county's population down to 639,300 on April 1, the date of this year's census, compared to 661,719 a decade earlier. County officials had expected a small increase in population.

By contrast, Prince George's population grew by 303,172 during the 1960s, an 85 percent increase that made it Washington's fastest growing suburb.

Although the preliminary data includes no information on the current age or race of the county's population, earlier state estimates indicated that the major movement of black families into Prince George's since 1970 had failed to offset a sharp loss of whites.

Earlier estimates also indicated that the bulk of the population loss occurred in older communities close to the District of Columbia line, while growth continued to Bowie and other sections of Prince George's outside the Capital Beltway. Detailed information on population changes in different parts of the county was not available last night.

The new preliminary figures were sent to Prince George's officials on Wednesday for comment and review before the Census Bureau issues its official count, probably in the early fall.

Stephanie Bolick, the county's public information officer, said Prince George's officials would not comment on the figures until a review is completed by the county's research and planning staff.

Despite the reported population loss, the census said the number of housing units in Prince George's rose by 16.9 percent over the past decade, from 200,637 to 234,612.

The paradox of fewer people living in more housing units occurred because average household size fell substantially from 3.34 persons per household in 1970 to 2.94 in 1980. This is part of a nationwide pattern, reflecting a major slowdown in births, an increase in divorce, and an upsurge in single people -- both young and adults and the elderly -- living alone.

Although the population count was about 30,000 less than county officials expected, the number of housing units was about 1,200 more.

A Prince George's planner speculated last night that the county may have underestimated the rapid decrease in household size but also may have failed to take note of illegal conversions -- probably homeowners who rent out basement apartments -- which the census takers had found.

Preliminary census figures reported last week showed an 18.6 percent population loss in Baltimore from 1970 to 1980 and a modest 8 percent gain in Montgomery County, although Montgomery' close-in suburbs, such as Bethesda and Silver Spring, lost substantially. Outlying areas such as Howard, Anne Arundel and Frederick counties grew rapidly.

Preliminary census figures have not yet been released for the District of Columbia or the major jurisdictions of Northern Virginia.

All the preliminary figures are subject to change, census officials warned, depending on the evidence local government present in an effort to prevent an undercount of their population.

Census figrues are the basis for distributing billions of dollars in federal grants.

According to the 1970 census, Prince George's County's growth during the 1960s was so rapid that it was exceeded by only one other large county in the United States, Orange County in southern California. The boom was fueled mainly by construction of moderate-priced apartments.

But in the early 1970s, construction in the county was abruptly halted by a sewer moratorium. Rental apartments became less profitable. In a shift spearheaded by former county executive Winfield Kelly, Prince George's changed its development policy, discouraging moderate-priced apartments and seeking -- with limited success -- to attract "a new quality" of development of expensive single family homes.

Development in Prince George's also was discouraged by a relatively high level of taxes compared to Northern Virginia and adjacent Maryland counties such as Charles and Anne Arundel.

Prince George's black population started growing rapidly in the late 1960s with enforcement of open-housing laws and the rise in black family incomes. The decrease in whites apparently was connected to the massive court-ordered busing program for school desegregation that went into effect in January 1973, and was accompanied by a sharp drop in white enrollment.

Demographers said the black families moving into Prince George's generally were smaller than the white families moving out.

Earlier census bureau estimates indicate that Prince George's population peaked in 1972 at 693,700, and had dropped steadily since then.