President Reagan yesterday tried to explain one of the admitted misstatements he made about unemployment at his Tuesday news conference, but added to the numerical confusion with an apparent slip of the tongue.

The president told about 1,500 political appointees assembled to celebrate the first anniversary of the Reagan inauguration that he was distressed by news reports he had misstated a series of numbers dealing with unemployment.

He defended his news conference claim that "there are a million people more working than there were in 1980." Reagan disagreed with those who pointed out that there were 508,000 fewer people working in December than when he took office last January.

"That isn't the way you use the figures," he said. Reagan said he was comparing the average employment for 1980 and 1981, adding the total employed for each year and then dividing by 12. This calculation, the president said, showed that an average of 97,270,000 people were working each month in 1980 and 98,318,000 in 1981.

But this explanation became confusing when he incorrectly said the difference between these two numbers was 148,000 instead of 1,048,000.

To compound the arithmetical tangle, deputy White House press secretary Larry Speakes later told reporters that the real 1981 figure was 98,313,000--5,000 less than the one the president used.

While Reagan and Speakes explained that the president was using an annual average--although he hadn't said so--at his news conference, Speakes said there was no explanation for where Reagan got the most politically sensitive of his several unemployment number mistakes: his allegation that the rising unemployment rate, which hit 8.9 percent last month, is "a continuation of an increase that got underway" in the last months of the Carter administration.

In fact, unemployment declined slightly from 7.6 percent to 7.5 percent to 7.4 percent in Carter's final three months and remained at 7.4 percent last January, the month of Reagan's inauguration. Then it declined to 7 percent last July before beginning the present rise.

Speakes offered no explanation for how the president got his information confused on the rates or for Reagan's use of two other inaccurate numbers for the average unemployment rates in 1980 and 1981: "I don't think we can assess any blame on those," Speakes said.

Administration officials said that the figures on total unemployment were sent to Reagan with other papers by Council of Economic Advisers Chairman Murray Weidenbaum just before the news conference. These same officials said the briefing papers did not include any inaccurate information.

Weidenbaum sent the president more papers yesterday assuring Reagan that the briefing information he had been given for his news conference was accurate and relevant, officials said.

These officials said that the use of annual unemployment averages is more "historically meaningful" than looking at the monthly fluctuations.

Reagan received enthusiastic cheers and whistles from the deputy secretaries, assistant secretaries, under secretaries and others assembled for the anniversary celebration, which began almost to the minute of his swearing-in.

He praised their work and told them they must keep up the fight against big government: "When you're up to your armpits in alligators it's sometimes hard to remember that you're here to drain the swamp," he said to the roughly 1,500 of his 2,200 top government appointees who attended.

"We have made an impressive start," the president said.

He appeared after many of his top advisers had warmed up the crowd with good-natured quips about their experiences during the first year.

While Reagan was reciting his triumphs, the Democratic National Committee rounded up a group of staff members, interns and students to provide an audience for a denunciation of the "unfairness" of the Reagan program.

In a "fairness day" press conference at Democratic headquarters that was echoed in 33 states, Senate Minority leader Robert C. Byrd (D-W.Va.) and D.C. Delegate Walter E. Fauntroy, a Democrat, read indictments of the Reagan economic policies.