A House subcommittee chairman accused the Federal Emergency Management Agency yesterday of making a "mockery" of federal law in arranging to test an emergency evacuation plan at the Shoreham nuclear power plant in New York.

Rep. Edward J. Markey (D-Mass.), in a letter to the new FEMA director, Julius W. Becton, said the agency improperly bypassed competitive bidding rules in hiring a consulting firm to help run a mock evacuation at the troubled Long Island plant.

Markey, who chairs the House Energy and Commerce subcommittee on conservation and power, accused the agency of "tortured" and "disingenuous" logic and called for the contract to be canceled. "FEMA's action is incredibly outrageous, and its so-called 'justification' totally preposterous," Markey said yesterday.

Becton's predecessor, Louis O. Giuffrida, resigned in September following several investigations of alleged mismanagement and contracting fraud at FEMA. The investigations produced evidence that FEMA was awarding too many noncompetitive contracts. At his Senate confirmation hearing in October, Becton said that such sole-source awards should be made only in emergencies.

The new contract comes amid a long dispute that has prevented the $4 billion Shoreham facility from obtaining a full-scale operating license. Such federal licenses cannot be granted without an evacuation plan in place. New York state and Suffolk County officials -- who oppose operation of the plant -- have refused to participate in the exercise, now planned for Feb. 13.

Court rulings have prevented the plant's owner, Long Island Lighting Co. (Lilco), from staging an evacuation on its own. The Reagan administration, which generally supports nuclear power, has tried to help Lilco win final Nuclear Regulatory Commission approval.

On Sept. 29 -- a day before the end of fiscal 1985 -- FEMA awarded the $81,250 noncompetitive contract to Theodore Barry & Associates of Los Angeles. Invoking a legal exemption, the agency said the situation was of "such an unusual and compelling urgency that the government would be seriously injured" unless normal bidding was bypassed.

In contract documents, FEMA officials said that Lilco is losing $1 million a day while the Shoreham plant remains out of service. If FEMA fails to provide technical assistance, the officials said, it "could result in a legal action being brought against FEMA by the utility" and the agency being charged for the daily losses.

Spokesman Bill McAda said FEMA would have no comment until a reply is sent to Markey. He said agency lawyers had "advised us that our actions were quite appropriate . . . There was no intention on our part to mislead anyone."

But Markey maintained that FEMA Associate Director Samuel W. Speck had misled him in a Nov. 1 letter, which said that no "substantial commitment" would be made to the Shoreham contract until more information was gathered. Barry & Associates had billed the agency for $36,562 the day before and the bill was paid soon afterward, Markey said.

Markey said it is hard "not to conclude that responsible FEMA officials knowingly and deliberately violated the law."

Fabian Palomino, special counsel to New York Gov. Mario M. Cuomo (D), said FEMA deliberately "concealed" the contract from state officials. "Clearly their objective is to open the plant," he said.

Lilco Vice President Ira Freilicher said the contract "was totally FEMA's idea" and that Barry & Associates had been critical of Shoreham. "There's nothing unusual about hiring a consulting firm in this connection," he said.

Douglas Bennett, vice president of Barry & Associates, said he knew of no other firm with such a record of working with the nuclear industry and helping plan evacuation exercises. "FEMA felt that we were sufficiently qualified to lend them some assistance," he said.