The administration will propose legislation to prevent major oil companies from buying other existing large companies unless they can show the purchase would enhance competition, the Justice Department announced yesterday.

John Shenefield, assistant attorney general for the Antitrust Division, said the legislation would bar acquisitions by approximately the 18 largest oil companies of other firms with sales or assets of $100 million or more "unless they can demonstrate real benefits to our economy."

"In the past these companies have sought to use their cash flow to buy department stores, mineral producers and circuses, diverting funds that might have been used for further energy development, Shenefield said.

"While the administration's proposal will not prevent all non-energy investment by oil companies it will help to reduce unnecessary diversions of the funds flowing to them as a result of (oil price) decontrol, he added.

The administration's proposal is "similar in concept" to a bill reported out of the Senate Judiciary Committee antitrust subcommittee this week sponsored by sens. Edward Kennedy (D-Mass.) and Howard tmetzenbaum (D-Ohio). That bill on which the full committee will hold a hearing today and other measures pending in the House, however, would bar all acquisitions of large companies regardless of any showing about competition.

Charles DiBona, president of the American Petroleum Institute, an industry organization, denounced the proposal, saying it was "completely out of step with the energy program outlined by the president earlier this week.... It is neither justified nor in the national interests."

Shenefield said the administration's approach would have prevented the purchase of Marcor, the Montgomery Ward holding company, by Mobil Corp. as well as that of Anaconda Corp., the copper producer, by Atlantic Richfield.

It wouldn't have stopped Exxon's move to create companies in the office equipment and information processing field. Asked about Exxon's recent purchase of 95 percent of Reliance Electric Corp. for more than $1 billion Shenefield hedged saying that might be seen as increasing competition among electric motor manufacturers.

The burden of proving that a particular purchase would enhance competition would be on the buyer; Shenefield indicated. That test well might allow the oil companies to continue to buy properties in other energy areas, including coal and nuclear, he went on to say. There is some sentiment on Capitol Hill to stop oil companies from expanding into other types of energy production.

Major oil companies generally defend their non-energy-related purchases as giving them needed diversification. They also say such acquisitions are dwarfed by their continued investments in energy.

A Treasury Department study covering essentially all large publicly owned corporations in the oil and gas industry - not just the top 18 - found that, from 1971 to 1977, the firms spent $8.1 billion on all acquisitions. The large integrated oil companies - those with refining and marketing operatiobns as well as oil and gas production - spent an average of only about 4 percent of their total cash available each year buying other companies, the study showed.

Meanwhile, the administration sent its "specifications" for a new Energy Mobilization Board to Capitol Hill yesterday in an attempt to get the concept incorporated in legislation that would speed construction of critical energy facilities.

The board would set deadlines for federal, state and local agencies to act on requests for required permits, and waive procedural, but not substantive, environmental safeguards.

The House energy and environment subcommittee, one of the recipients of the detailed outline, was unable yesterday to agree on a version of a socalled fast-track bill with the same goal of speeding construction of energy facilities.

The subcommittee chairman, Rep. Morris Udall (D-Ariz.), thought that agreement had been reached on a bill that could be reported to the full Interior Committee to serve as a basis for debate while considering a number of changes, including those proposed by the White House.

However, Rep. Steven Symms of Idaho, the ranking Republican, objected to the procedure, and no action was taken.