THERE IS a superficial logic in the plan Secretary of the Interior James Watt has proposed for the national parks system. He wants to fix up the parks that already exist and halt, at least temporarily, the acquisition of new park land.Given the condition of many of the great national parks -- shabby is the right word -- and the vast expansion of the park system in the last decade, the idea seems to make sense.

As with many such ideas, however, there is a problem. The government has a backlog of about a billion dollars' worth of land that Congress has said it wants to buy for the park system. This includes not only many recently authorized parks but parts of such established parks as Redwood and the Cape Cod National Seashore.

The people who own that land are mostly waiting to get their money; there isn't much of a market for land the government has said it is going to take. Under Mr. Watt's proposal, these owners will simply have to continue to wait -- which, in some cases, will make them happy -- while the value of their land appreciates. The ultimate cost to the taxpayer, of course, is likely to be much higher in 5 to 10 years than it is now.

Mr. Watt is right in claiming that there is a need to reevaluate the whole park system. The "park barrel" did seem to replace the old pork barrel in recent sessions of Congress when members began to prefer parks to dams in their home districts. But the way to handle that reevaluation is for Mr. Watt and his experts to recommend to Congress which parks should be de-authorized.

In the meantime, it would be wiser to spend whatever money is available buying land the Park Service knows it will need than to scrub the buying program in favor of refurbishing the older parks. The public has put up with the shabbiness of some of those areas for a good many years now and can continue to do so for a while longer. Any appreciable delay in the acquisition program increases the risk that the land will never be bought and that it will be used in ways incompatible with a park before the government gets around to obtaining it.