Donald T. Reagan, President-elect Ronald Reagan's choice as Treasury secretary, predicted yesterday that inflation next year will be "lower than it is currently if the Reagan economic package can be put in.

"There can be no definite forecast as to how far down, but it will be coming down," he said.

Reagan also said he expects the economy to be sluggish in the first half of 1981. Analysts at Merrill Lynch & Co., the holding company of which he is chairman, believe the commercial bank prime lending rate, which reached 20 percent this week, will rise further before it begins to come down, he said in an interview on NBC'S Today's Show.

Meanwhile, the Federal Reserve reported some news that could relieve some pressure in financial markets. The measure of the nation's money supply known as M1-A fell by $200 million to a total of $388.7 billion in the week ended Dec. 3. More importantly, the previous week's figure for M1-A, which includes currency in circulation and checking deposits at commercial banks, was revised downward by $700 million. M1-A, which also includes checking deposits at other financial institutions was unchanged at $413.9 billion last week but revised downward by $800 million the week before.

The Federal Reserve, in an effort slow growth of the money supply, has been tightening its grip on the expansion of credit in the face of businesses' sharply increasing demand for funds. Its actions have helped push up interest rates.

The latest revisions suggest the Fed policy is having an impact on money supply growth. At $388.7 billion, M1-A is up only $100 million in the latest four-week period, and M1-B, at $413.9 billion, is up only $600 million over the same period.

The poor outlook for economic activity and inflation, as well as record levels for interest rates, led David Stockman, Reagan's choice as director of the Office of Management and Budget, to declare in the same interview, "We're going to inherit a situation that is ather dismal.

"But on the other hand," Stockman added, "if we can put a package together that is credible, I believe we can pull out of it and begin the path of expansion and reducing inflation. I would hope by 1982 the indicators we've seen in 1980 -- high interest rates, booming inflation, stagnation and rising unemployment -- are all heading in the other direction. We have confidence it can be done."

The reagan package will include major cuts in personal and business taxes as well as large reductions in nondefense spending programs.

"What we intend to do is to have an economic package that, to use an old Marine Corps expression, starting on Jan. 20 we want to hit the beach running with this economic policy program and put it into effect quickly," Reagan said. "That in itself will start to make inflation abate, and as inflation starts down definitely the prime rate's going to come down."

Meanwhile, the Commerce Department reported that the value of business inventories rose a modest 0.6 percent in October to a seasonally adjusted level of $457.3 billion. The rate of increase was the same as in September. Most analysts believe businesses have been accumulating inventories involuntarily in recent weeks as sales have not kept pace with production.