U.S. District Court Judge Milton Shadur of Chicago yesterday accepted a voluntary public school desegregation plan that was proposed by the Chicago School Board and supported by the Justice Department.

The plan, which does not call for any mandatory busing, would be based on a system of magnet schools, combined with voluntary transfers and redrawing of some school districts to increase the racial mix.

The Chicago system, which includes about 600 schools, is the nation's third largest and has been plagued by financial difficulties in recent years.

About 61 percent of the city's nearly 436,000 students are black, with about 16 percent white, 20 percent Hispanic and the remainder Asian or American Indian.

After the lawsuit was filed in September 1980 under the Carter administration, the Chicago School Board entered into an agreement with the Justice Department to complete a desegregation plan by March 1981. One of the requirements of that plan, as outlined by the department at the time, was a backup proposal that included mandatory busing and student reassignment if voluntary measures did not work.

After President Reagan took office the Justice Department initially opposed a plan by the school board similar to the one approved yesterday. But in August, 1981, the department changed its position, announcing its general approval of the board's plan.

Although the Justice Department and the court retain the power to monitor the Chicago desegregation plan, there is no backup plan for mandatory measures if it is not successful.

The plan defines a "desegregated" school as one that contains at least 30 percent each of minority and white enrollment. Shadur said that those percentages meet the minimum test of constitutionality. The judge also said he is not troubled by the fact that blacks and Hispanics are lumped together under the plan.

The Reagan administration has been outspoken in its opposition to mandatory school busing as a school desegregation remedy.

William Bradford Reynolds, head of the Justice Department's Civil Rights Division, has said he opposes mandatory busing in new desegregation plans and is willing to ask the courts to overturn existing busing plans in cases where there has been white flight and local officials appeal to him for help.

Some civil rights lawyers have complained bitterly about that change in policy, charging that the federal courts have ordered mandatory busing only as a last resort in cases where voluntary measures have failed.

Reynolds yesterday called Shadur's decision "extremely encouraging," and said he hoped it would encourage other courts to consider "similar voluntary measures to achieve desegregation where the school board has demonstrated a similar enthusiastic commitment to make the plan work . . . . The court found the plan to be clearly within the broad range of constitutionally acceptable remedies."

"The effectiveness of the voluntary desegregation effort in Chicago will . . . depend on the continued support of the school board and the community," Reynolds said. "We remain confident that the proper implementation of the plan . . . can achieve more lasting desegregation than a mandatory student reassignment plan."

Reynolds said the Justice Department will "carefully monitor" implementation of the plan to be sure it "actually measures up to the court's expectations."

Alexander C. Ross, the department attorney involved in the case, said the plan has been partially implemented and will be reviewed by the school board each March. He said the court and the department will continue to monitor the case and could reenter it if there are problems.

"This court will not abdicate its constitutional responsibilities" by yesterday's approval, Shadur said. "To a major extent, the plan reflects a promise of things to come. That promise is within the range of constitutional acceptability if it is kept. As both the decree and the parties expect, this court retains jurisdiction to make certain that takes place."