President Reagan, preparing to veto a bill that would create 50,000 acres of wilderness forests in Florida, yesterday signed into law a similar measure protecting more than 35,000 acres of wild forests in West Virginia's popular Cranberry Backcountry.

Administration officials said the Florida veto is not a stand against wilderness protection but rather an attack on other features of the bill, which budget advisers viewed as a bailout of phosphate companies seeking leases in Florida's Osceola National Forest.

Meanwhile, Interior Secretary James G. Watt announced that he will protect the Osceola by denying the four firms permission to strip-mine the boggy forest, which spreads across the Florida panhandle.

Efforts to mine the Osceola have generated angry opposition across the political spectrum in Florida. The Florida wilderness bill, co-sponsored by the full state delegation, would have permanently banned all mining and timber cutting in the Osceola, while requiring the government to compensate the four firms for the value of phosphate deposits in the forest.

But while the administration sought to portray both the Florida and West Virginia decisions as friendly to the environmental movement, conservation groups criticized the expected Florida veto as "unwise" and "dumbfounding."

Also yesterday, six environmental groups filed suit against Watt in an effort to prevent him from dropping wilderness protections from 805,000 acres of wild western lands being studied for inclusion in the federal wilderness system.

Watt said last month that the acreage does not qualify for such protection.

"We are forced to conclude that this administration is anti-wilderness," said Hope Babcock, an attorney for the National Audubon Society, one of the groups suing Watt. "Couple the Florida veto with the fact that we have to file suit to protect wild lands that the government should be protecting and what else can we conclude?"

However, Florida politicians who have been trying for more than a decade to prevent phosphate mining in the Osceola said they were somewhat satisfied with the arrangement.

They also said Reagan's aides have promised that the administration will support a wilderness bill this year if it excludes the provision compensating the phosphate companies.

"I'm very pleased with Secretary Watt's decision to deny the leases" to the four firms, said Sen. Lawton Chiles (D-Fla.). "I hope it's intended as a permanent solution."

Administration officials contended it could have cost up to $200 million to compensate the four phosphate companies, Kerr McGee Corp., Monsanto Chemical Products Co., Global Exploration Co. and Pittsburgh and Midway Coal Mining Co. "It was a rotten deal," one of them said.

However, lawyers for environmental groups disputed this, saying that Watt's ruling has rendered the companies' claims worthless. In denying the leases, Watt concluded that the Osceola is too sensitive to be restored after strip mining, making it ineligible for phosphate leasing.

The new Cranberry Wilderness becomes the second largest wilderness forest east of the Mississippi, after Minnesota's Boundary Waters Canoe Area. It requires the government to compensate the Chessie Corp. for an estimated $30 million of coal deposits beneath the lush forests of yellow birch, maple, black ash and oak.