The Justice Department reversed its opposition to the Chicago school board's controversial desegregation plan yesterday, giving it an interim stamp of approval and delaying judgments on some critical issues until December.

Just a month ago, department lawyers complained to a federal judge in Chicago that the same plan it found acceptable yesterday was so incomplete in several crucial areas that it shouldn't be accepted.

The action yesterday appeared to reflect the Reagan administration's publicly-stated opposition to busing because it makes no change in the Chicago board's plan to delay any consideration of mandatory busing for at least two years while voluntary remedies are tried.

But William Bradford Reynolds, assistant attorney general for civil rights, said yesterday that the department's reversal didn't mean it was dropping its concerns.

Reynolds said his attorneys had misunderstood some school board positions because of a lack of information, and that the board was now committed to changes that resolved some of the department's concerns.

Robert C. Howard, an attorney for the Chicago board, hailed yesterday's joint statement as "extremely significant" because it said for the first time that the government found the board's basic policy direction constitutional.

He said he didn't feel that yesterday's statement signaled any "tremendous policy change" at Justice. The reluctance to use busing expressed in the statement is no different than the last resort language used when a consent decree in the dispute was signed last fall during the Carter administration, he said.

The Chicago school system is the nation's third largest and has been plagued by financial difficulties in recent years. About 61 percent of the city's 450,000 students are black, with about 18 percent each white and Hispanic.

Thomas Atkins, general counsel for the NAACP in New York, said yesterday that he viewed the announcement as an attempt by the city to get federal funding from the Department of Education, and said his group will oppose it.

The Reagan administration has vocally opposed busing in school desegregation cases, with Reynolds and Attorney General William French Smith stating their intent to try other remedies instead.

Just Thursday, Reynolds decided not to appeal a court order that dismissed the department's efforts to force cross-district desegregation between Houston and its suburbs.

In the Chicago case, the board has proposed using magnet schools, voluntary transfers and redrawing some school boundary lines in an effort to mix black and white students without busing.

Yesterday's statement noted that there had been "significant lapses of communication" between the two sides before Justice filed its critical response last month. The two sides now agree, the statement said, that the acceptability of the school's plan can't be judged until the board files a student assignment plan in December.

In the meantime, Justice approved the board's current efforts to desegregate the city's schools. For instance, the city agreed to change the boundary lines on some high schools in an attempt to draw more minority students this fall. The board also projected that its programs this fall would result in the voluntary transfer of 5,100 minority students.

A previous concern that the board might be setting a 30 percent minority cap on its schools has been resolved, yesterday's statement said, because the board's attorneys agreed that was not a policy.

Both board attorney Howard and Justice case attorney Alexander C. Ross said the city had "a long way to go" before it met all the requirements of the consent decree. But they also agreed that substantial progress was made in the six weeks since Justice filed its critical response.

In its July filing, the department criticized the board's plan for not providing mandatory backups in case the voluntary measures to desegregate don't work. Yesterday's statement gives the school board more leeway.

Reynolds said he feels Justice and the board's lawyers now will work more closely so "we won't have a showdown in December" when the rest of the board's plan is in place and the results of this fall's voluntary programs are considered. "We want to keep apprised as they develop things," Reynolds said. "We'll be looking over their shoulder."

In the statement, the parties said they are "encouraged by the positive tone and constructive results of their discussions in the last month. Significant concerns have been allayed or resolved . . . . "