Maryland Gov. Harry Hughes, in the first Cabinet-level change since his reelection to a second term, is expected to appoint Ruth W. Massinga today as secretary of the massive Department of Human Resources.

Sources said Hughes' appointment of Massinga, who now overseas the social services section of the agency, is scheduled to be announced at a press conference.

Massinga, 42, of Bethesda, would be the second woman and second black to fill a Cabinet-level post in the Hughes administration. Legislative officials also believe Massinga would be the first black woman ever to hold a Cabinet-level job in the state.

The department, one of the largest in state government, comprises welfare, unemployment, adoption, foster care and other social programs, and as such is the focal point in Maryland for Reagan administration cutbacks.

The current secretary, Kalman R. (Buzzy) Hettleman, who earned widespread praise as a fierce advocate for the poor and as a tireless secretary, told Hughes several months ago that he was resigning at the end of the year because of fatigue and frustration with the Reagan administration.

Massinga, reached at home yesterday, said she had received no official word, although the governor's appointment secretary called her to say Hughes or a member of his staff would be in touch soon.

Massinga reportedly was selected from among several candidates after a decision by Hughes to appoint Barry Von Lear, an official at the National Governors Association, fell through.

Massinga's anticipated appointment is the first of three or four expected changes in the 12-member Cabinet.

Thomas Schmidt, secretary of the Department of Public Safety and Correctional Services, has told the governor that he is considering several other jobs and will likely leave soon.

Natural Resources Secretary James B. Coulter recently announced his retirement. Transportation Secretary Lowell K. Bridwell, who was recently the target of an unflattering investigation into his past conduct as a consultant in New York, is reported to be considering leaving the government.