Carl B. Williams, a consultant who has specialized in helping states reduce their welfare rolls and uncover fraud, has been hired by the Social Security Administration at $245 a day to review procedures in the $7.8 billion Supplemental Security Income (SSI) program.

Williams has a reputation for being a tough advocate of "workfare" and various measures to tighten up programs. Thus, his appointment to review SSI, the federal program for impoverished aged, blind and disabled people, has worried some welfare advocates here.

Robert Greenstein, administrator of the food stamp program during the Carter administration and now director of the Center on Budget and Policy Priorities, said Williams "never has been known to let the facts stand in the way of his efforts to attack low-income programs."

This is the second time in recent months that John A. Svahn, now undersecretary of Health and Human Services, has hired Williams as a consultant. Last November, Svahn hired him to work on SSI, but after three weeks Richard S. Schweiker, then secretary of HHS, ordered him fired.

Schweiker contended that Williams' job violated a hiring freeze. Also, Schweiker reportedly feared that Williams' other consulting jobs might create the appearance of a conflict of interest.

Svahn again hired Williams June 6. Williams denies there is any conflict of interest, and Svahn has said that the appointment has been approved by the Social Security Administration's personnel department.

Williams is an old hand in the welfare business. He worked under Svahn in the California welfare department when President Reagan was governor. While in California, he also worked with Robert B. Carleson, now White House adviser on welfare and domestic programs.

Later Williams was deputy U.S. commissioner of welfare under Carleson from 1973 to 1975. Then, he and Carleson worked together as welfare consultants, advising communities and states as well as the transition team of Massachusetts Gov. Edward King (D) four years ago.

Most recently, Williams advised the Massachusetts welfare department under King on using computers to discover if welfare clients had hidden bank accounts. This year he advised the Agriculture Department on proposed changes in food stamp regulations.

Both he and Svahn said that Williams will not work on substantive policy matters but on ways to improve procedures, reduce paper work and verify incomes of SSI recipients. Williams said that is similar to the work he did on food stamps, "trying to simplify and clean up regulations . . . streamline the stuff."

Svahn said that he hired Williams after reading stories in The Washington Post about Williams' work in Massachusetts. "We spend $15 million to $16 million in the SSI program verifying bank accounts," said Svahn, "and you know how we have to do it? By phone."

Williams said that his views on welfare are irrelevant to his SSI assignment because government officials "have to follow the law."

However, Nancy Amidei, deputy assistant secretary of HHS for welfare legislation under Carter and now director of the Food Research and Action Center, said, "We know from past experience that his views do get in the way of sound and objective procedure and, for whatever reason, invariably result in harm to people in need who require help from public programs."