The Montgomery County Board of Education, led by its conservative majority, is expected today to adopt a key policy change that will curtail its efforts to achieve racial balance in the county schools.

The proposed change would essentially alter the definition of when there are too many minorities in a particular school so that the board would not have to act as frequently to try to remedy the imbalance. The result would be a greater concentration of minority students at schools in the southeastern part of the county.

It has been attacked as a step backward in the integration effort, and cited as additional evidence of a hostility to the concerns of minorities at a time when racial incidents are increasing in a county known for progressive views. But board members who support the change in policy brush aside charges that they are deserting minority concerns, saying the charge reflects only the increase in minority populations, a new emphasis on neighborhood schools and a distaste for busing.

"I think we're being realistic," says school board member Marian Greenblatt. "Housing patterns in certain neighborhoods have changed. The day of mandatory busing is over. We're saying we favor neighborhood schools with further opportunities for integration. The only other option is busing children across county to Potomac, Olney, Damascus and Gaithersburg. I don't think that's what the community wants."

Since 1975 when it adopted a Quality Education/Racial Balance policy, the school board has committed itself to seeking methods to adjust racial imbalances when the minority population in a school exceeds 50 percent.

Under the proposal scheduled for a vote this afternoon, the board would only consider ways to adjust imbalances when minorities exceeded by 40 percent the proportion of minorities in the whole school system. According to school department figures, minorities now constitute 22 percent of school population, double their percentage five years ago.

With a "floating figure" a school then would have to be more than 62 percent minority before the board would address the question of imbalance. And if the minority population continues to climb in the county, so would the allowable percentage of minorities in county schools.

There are now 21 schools with a racial imbalance in the county. Under the board proposal there would only be seven, according to school planners. The superintendent who is now examining ways to lessen the racial imbalance at Montgomery Blair High School would not be obliged to do so under the revised policy.

For years overwhelmingly white, Montgomery County, like all of Washington's suburbs, changed markedly during the 1970s. During that period, the percentage of black residents grew from 4.1 percent in 1970 to 8.8 percent in 1980, while Asians and other minorities increased from less than 1.5 percent to nearly 6 percent.

Despite marked increases in minority populations, the county school system as a whole is facing a sharp decline in enrollment and plans to close 34 schools in the next five years.

Many of the most dramatic demographic changes have taken place in the Silver Spring and Takoma Park areas. For example, the percentage of minorities in the county's 22 high schools exceeds the county wide percentage in just five high schools, but all of them are in the southeastern part of Montgomery.

Proponents of the change say they habe the votes to pass it but they have been sharply criticized.

"The conservative members of the board are abandoning desegregation and they're doing it because the political arithmetic is on their side," charged Blair G. Ewing, one of the last two liberals left on the board. "The prime purpose of the policy was to avoid racial isolation. This resolution and other steps the conservatives plan to take will lead to the creation of an educational ghetto. What the board is saying is "We don't give a damn.'"

And Roscoe Nix, a former member of the school board and currently president of the county chapter of the National Association for the Advancement of Colored People, criticized the board's proposed change.

"It's a reversal of efforts that have been made by the community at large. Its consequence will be to advertise to the rest of the nation that Montgomery County is primarily interested in white folks and to inflame the prejudices of whites already hostile to blacks."

Greenblatt defended the board's commitment to integration and to minority students, saying, "We're spending $14 million on disadvantaged and minority students, over and above the per pupil allocation of $3,000 a year. People are getting a little hysterical. We are tired of using children as pawns in these statistic games. We have to stop manipulating the child. We're concerned with the education the child is getting in the classroom."