The U.S. Civil Rights Commission said yesterday that black family farmers are a rapidly diminishing group in this country and could disappear almost entirely by the end of the century.

All family farmers face problems and have fallen in number, but the commission said blacks face special problems of racism and a less-than-fair share of available credit, including supposedly last-resort credit supplied by the Agriculture Department.

The commission said in a 196-page study, "The Decline of Black Farming in America," that the number of black farmers fell 57 percent in the last decade, about 2 1/2 times the rate of decline for white farms.

There were 926,000 black-run farms in 1920, and about 57,000 by the end of the 1970s. There will be 10,000 by the end of the century if present trends continue, the commission said. There were about 5.5 million white farms in 1920 and there are about 2.3 million now, it reported.

Outgoing commission Chairman Arthur S. Flemming called on the Reagan administration to intervene immediately to help black farmers, saying that the most pressing need of black farmers is credit.

He was especially critical of the loan policies of the federal Farmers Home Administration, which is supposed to be the lender of last resort for farmers.

Despite what the commission said was their disproportionate need, black farmers got only about 2.5 percent of the total dollars that the FmHA lent in the fiscal year that ended last September, the commission reported.

In one program, "limited resources loans," which Congress created expressly to benefit minorities, a smaller percentage of black farmers got loans than whites in some states, the commission said.

The commission said its inquiry indicated that the FmHA may be involved in the very kind of discrimination it should be seeking to correct. It said it had found in some instances that the federal agency was part of the problem, not the solution, and that community groups and black farmers generally held a similar view.

The commission urged that congressional oversight hearings be held to determine the extent to which Agriculture Department policies have helped or hindered black farmers. It also asked that the Agriculture secretary put in place a program specifically aimed at assisting minority farmers, and urged tougher implementation of civil rights policies and regulations.

"Really, what we're asking for basically is a vigorous implementation of existing law," Flemming said. He appeared to doubt that the commission's recommendations would be taken seriously by the Reagan administration, saying he hoped the report would spur a public reaction that would then be brought to bear on elected officials.

"If the administration is as interested in economic development as they say they are . . . this would be an object of first concern," said commission Vice Chairman Mary F. Berry.

The commission said FmHA officials had been given the opportunity to review portions of the report released yesterday. The federal agency had no immediate response.

The reasons for black migration from land in the South to the big cities over the last 60 years are varied and complex. In the 1930s and 1940s there was the pull of better jobs, better opportunities for education and a racial environment in the industrial North that was less hostile.

In the 1950s and 1960s, black farmers and white small farmers found it increasingly difficult to compete with the growing number of large mechanized farms, which could cut costs by substituting newly marketed farm machinery for labor.

The commission noted that the loss of black-owned land is being played out at a time of rising land values and industrial growth in the South.

"The loss of this land and the inability of blacks to endure as landowners may result in serious consequences for racial relations in this country," the agency warned.

"A society where whites control virtually all agricultural production and land development . . . is not racially equal. Such an imbalance can only serve to further diminish the stake of blacks in the social order and reinforce their skepticism regarding the concept of equality under the law."