Richard T. Pratt resigned yesterday as chairman of the Federal Home Loan Bank Board, the regulator of the nation's savings and loan associations. His departure, set for the end of April, has been anticipated for several months.

His successor is expected to be Edwin J. Gray, a San Diego savings and loan executive and a long-time associate of President Reagan.

Gray's nomination is being held up by the Arkansas congressional delegation in an effort to prevent a transfer of the Federal Home Loan Bank from Little Rock to Dallas.

Pratt said yesterday he felt the S&Ls in the affected district should have the major decision in where to locate the bank and that the FHLBB in Washington should reverse the local board's decision to move to Dallas only "for good and substantial reason." A vote is scheduled in about two weeks.

Pratt said he resigned for personal, financial and career reasons. He plans to return to Salt Lake City to work in the private sector.

When Pratt was appointed two years ago, the savings and loan industry was, in his words, "on its deathbed" because of the adverse effects of high interest rates and the industry's "obsolete structure which destined it to oblivion." Pratt was instrumental in drafting legislation allowing S&Ls to expand their powers. These, together with a drop in interest rates, are easing the industry out of its worst crisis since World War II.

Nevertheless, 400 troubled S&Ls will be eligible for assistance this year. If one-third of those apply for aid, as expected, the government's potential liability could be $170 million.

Asked yesterday at a press conference why he was resigning before the end of his term, Pratt responded, "I had a specific agenda. I have contributed what I could contribute. I don't know what the optimum time for a government administrator is. I suppose it's long enough to accomplish something and be effective, but not so long that one starts to believe one's own press releases."

Pratt listed four priorities for his successor: a better accounting system to show the true financial situation of S&Ls, an overhaul of the Federal Savings and Loan Insurance Corp., the need to impose market discipline on S&Ls and a crackdown on management for abuse of fiduciary duties.

He warned against consolidating overlapping banking agencies into one super regulator in the name of efficiency, a proposal being explored by a vice presidential task force.

"The cost of regulations in running an agency is minuscule compared to what we can do in a more global sense. We may cause either more benefit or more harm than our yearly operating budget. It is incorrect to look at regulatory agencies based simply on activities they perform. The real question is whether they make markets efficient for consumers."