Interior Secretary James Watt plans to spend $2,000 in tax money to partly reimburse a National Park Service fund that financed two controversial private Christmas parties he sponsored at Arlington House, also known as the Custis-Lee Mansion, an Interior official said yesterday.

Disclosure that Watt had paid for the two parties with money from a fund composed of voluntary donations and that one of the parties had forced closing the public mansion, the home of Confederate General Robert E. Lee, angered donors to the fund and volunteers who run free tours of the house.

The two parties--a breakfast given by Watt's wife for cabinet wives and a cocktail party Dec. 17 for about 150 guests--were described by Interior officials in December as private functions.

The department disclosed yesterday that the total cost for the cocktail party, attended by Presidential Counselor Edwin Meese and others, was $5,176, including $2,323 to rent a tent that was pitched outside the mansion, which overlooks Arlington National Cemetery.

The cost for the breakfast, attended by the wives of Vice President Bush, CIA Director William Casey and others, was not available.

Richard Hite, deputy assistant secretary of administration for Interior, defended Watt's use of the voluntary donations fund, saying that Congress did not appropriate funds for the department until after the fiscal year had begun. Government entertainment funds typically should pay for such parties, he said.

"We have made the determination to repay some of what we have used," Hite said, "but we used entirely legal discretion in using the fund . . . ."

Watt would repay more, but to do so would again deplete the secretary's entertainment fund, Hite said. Hite, who oversees the voluntary donations fund, said yesterday it was common practice to use it to pay for half of the entertainment expenses. "We don't have enough," he said.

A report listing expenditures from the voluntary fund for June, July and August of 1981 show that Watt spent $1,373.75 for breakfast and luncheon meetings with congressional officials and public interest groups. A record of those meetings shows that representatives of coal, petroleum, mining, oil pipeline associations, the National Rifle Association and conservation groups attended.

The private fund is composed of donations from a network of private, nonprofit groups at national parks whose volunteers produce educational materials and sell them to park visitors. The groups routinely donate part of their money, anywhere from $40 to thousands of dollars, to the fund. It is supposed to be used to finance publishing and educational seminars.

"I think its absolutely despicable that those funds would be used for that," said David Thornton, chairman of the Everglades Natural History Association in Florida. "This is the last thing in the world that money should be used for."

"I personally don't think Christmas parties to butter up some public officials . . . is proper use of the fund," said Otis Swisher of the Crater Lake Natural History Association in Oregon.

Eleanor Inskip, business manager of the Canyonlands Natural History Association in Utah, said her association will withhold its donation this year until "we have a board meeting and discuss this matter."