The Interior Department is preparing legislation enabling a Rockefeller-owned company to place two of North America's poshest vacation resorts at the service of the national park system, Interior Secretary James G. Watt, who is enthusiastic about the proposal, said yesterday.

The idea was proposed by Laurance Rockefeller and would bring the two resorts -- the tropical Caneel Bay Plantation in the American Virgin Islands and the mountain retreat at Jackson Lake in Wyoming's Grand Teton mountains -- closer to the fold of the National Park Service. They are owned by a Rockefeller company, Rockersorts, Inc.

Under the proposal, first put forth by Rockefeller during the last year of the Carter administration, the two resorts would be placed under the aegis of a new Public Benefits Corp., a quasi-public corporation.

The corporation, Watt told The Washington Post, hopefully could turn a profit which then would be used for "building and meeting the needs" within existing national parks. It would not distribute profits to stockholders. Nor would the properties become true wards of the National Park System.

Profits from operation of the resorts could be used to maintain and upgrade parks throughout the national system, which Watt has been pushing aggressively on Capitol Hill. Watt has recommended a moratorium on new parkland purchases. He wants Land and Water Conservation Fund monies, now set aside for purchases, diverted to upkeep instead.

A Public Benefits Corp. would need congressional approval. But Watt said "we hope Congress will create" the corporation and Douglas Baldwin, assistant secretary for public affairs, said legislation was "in the works" at Interior.

Caneel is a 150-acre luxury resort on St. John's on the edge of Virgin Islands National Park. The Rockersort at Jackson Lake is located on park land in Grand Teton National Park. In both cases the Rockefeller family originally purchased the land for the parks and then gave it to the government, building the resorts as luxurious adjuncts to the parklands.

Baldwin said the proposed Public Benefits Corp. originally would include only the Rockefeller assets at Caneel and Jackson Lake. If the concept works and produces resources for upgrading the park system, however, he added, of other properties "could be accepted by the Public Benefits Corp. in the future."

Baldwin acknowledged that the proposal offered major tax benefits to the Rockefeller corporation but said the "tax benefits should not denigrate the magnitude of Laurence Rockefeller's gifts to the public."

Watt described the proposal as a "multimillion-dollar gift . . . a whale of a gift."

In New York, Rockeresorts Inc. president Richard T. Holzman estimated the value of the Caneel resort at about $20 million. The value of the Jackson Lake holding, he said, was more difficult to estimate because Rockeresorts owns no land outright at the Wyoming site.

Holtzman said it was his understanding that, under the proposal, Rockeresorts would remain as the concessionaire at the two vacation spas. He also said the nature of the resorts, known both for their luxury prices and their attempt to maintain the natural environment of the areas, would not change substantially.

The resorts have had trouble maintaining high quality and also earning profits, however.

Holtzman said the purpose of the proposal was to ensure that the lands and resort facilities do not "fall into other hands, commercially, at some time."

He said Caneel and Jackson Lake were not for sale. Rockeresorts owns five vacation properties, including a golf course in the American Virgin Islands, Little Dix Bay Resort in the British Virgin Islands and the Woodstock Inn in Vermont.

In 1979, the corporation sold a prime tropical resort at Mauna Kea in Hawaii to UAL Inc., the holding company that controls United Airlines and Western International Hotels.