For more than 40 years, the National Park Service has been trying to buy 2,000 acres within the Great Smoky Mountain National Park from private landowners who didn't want to part with the tract.

But now that the current owner, Cities Service Co., is willing to sell the land, the Interior Department has put the deal on hold because it has changed its policy on acquiring park land.

Since he took office, Interior Secretary James G. Watt has argued that the National Park Service should take better care of what it already owns, rather than simply acquire more land.

Now the department has moved to scrap its "park acquisition plans," which were developed by the Carter administration to guide the purchase of congressionally designated land within or next to 140 of the 333 units of the national park system.

Instead, Interior will develop "land-protection plans" over the next three years to specify ways in which federal park land can be protected without purchasing it.

"The primary effect of the new policy is that alternatives to cash purchase . . . are being more fully explored and employed wherever practicable," said National Park Service Director Russell E. Dickenson.

The new protection plans will explore cheaper ways of protecting the land, such as working out agreements with private landowners over the use of their land or asking local zoning boards to restrict the land's use. The plans also would consider cheaper ways of acquiring land, such as getting another agency to transfer it to the park system or soliciting donations to purchase a tract.

Of the 79.4 million acres of land in the national park system, 3.3 million are privately owned and another 1.3 million are controlled by state and local governments. One of every 17 acres belongs to someone other than the federal government.

"We're no longer in the position of having unlimited funds available for park acquisition," said Will Kriz, chief of the park service's land resources division. "Still, Congress has put the burden on the secretary to protect the park lands it has designated."

The first group of protection plans, which are scheduled to be completed by Sept. 30, will cover park units that Congress ordered acquired in fiscal 1982 and 1983, and those facing the threat of development. "This is not going to be a question of trying to determine what the owners' wishes are," Kriz cautioned. "It's the cultural resources that need protection and we're simply looking to expand the alternative ways to do that."

The new approach, though, has drawn a mixed reaction from Capitol Hill.

Frank D. Cushing, staff director of the Senate Appropriations subcommittee on interior, said Watt's staff has come to realize "that they can't afford to purchase everything we have identified for acquisition."

Cushing noted that over the years Congress has authorized the purchase of more than $2 billion worth of park land, but for one reason or another, Interior has not been able to spend it as fast as Congress would have liked.

"There's no question that there are a few things out there that we should really acquire," Cushing said. But, he added, "I would doubt very seriously that there's very much that needs outright purchase. The integrity of the park can be the same, regardless of how it is protected."

House aides, however, are more skeptical. "Their avowed interest is in not acquiring interest in any more lands," said a House Appropriations Committee aide. "Watt will dance with you all day long on whether there is a moratorium on land acquisition or a moratorium on the creation of new parks." The proposal for protection plans, he said, could be viewed as "a deliberate effort to slow down the process."

If saving money is the goal, said Dale Crane, a staff member of the House Interior subcommittee on public lands and national parks, then "Watt is off course. Easements don't always save money. In easements, the federal government may pay 80 or 90 percent of the full value" of the land without assuming ownership, and sometimes gaining control for only limited time.