A federal judge yesterday halted implementation of Labor Department regulations that union officials say would have weakened a law setting minimum wages on federally financed construction projects.

The new rules affecting the 1931 Davis-Bacon Act were supposed to take effect next Tuesday.

U.S. District Court Judge Harold H. Greene said in a 17-page opinion that the department's proposals amounted to an abrupt change "of a longstanding administrative position" regarding the act.

"When an agency abruptly changes a longstanding administrative position . . . it may be expected at a minimum to show that the earlier understanding of the statute was wrong or that experience has proved it to be defective. . . . The Secretary of Labor Raymond J. Donovan has done neither," Greene said.

The judge said implementation of the regulations under those circumstances would hurt workers who "would be forced to accept lower wages--a change for which they will have no legal avenue or redress."

The Davis-Bacon law--originally designed to block the use of cheap, presumably unskilled, labor on federal projects by unscrupulous contractors--sets a "prevailing wage" determined by the Labor Department on a job-by-job basis.

Many construction industry officials and their supporters in Congress had criticized the law as being obsolete and unduly inflationary, by as much as $1 billion annually.

The Reagan administration proposed changes that it said would reduce federal building costs under the Davis-Bacon Act by $600 million a year. Those changes essentially would have redefined "prevailing wage" and would have allowed contractors to hire more lower-paid "helpers" in proportion to the number of top-scale journeymen on federal jobs.

The suit against the proposals was brought by the Building and Construction Trades Department of the AFL-CIO.