The Army will shrink to a 10-year low of 772,600 soldiers in this budget year and may drop to pre-Korean war levels to free money needed to pay for weaponry it ordered during the boom days of President Reagan's defense buildup, Army leaders said yesterday.

James R. Ambrose, who leaves his post as undersecretary of the Army this month, told The Washington Post that, if zero-growth defense budgets continue, as seems likely, he would rather let the Army have as few as 521,000 soldiers than try to field a large force without arms needed to combat the Soviet threat.

With money saved through personnel cuts, Ambrose said, he would keep buying today's tanks and planes while developing for the 21st century a new generation of weaponry, including robots for driving tanks and clearing mine fields.

"The idea is to keep people off the battlefield, to keep them alive," he said in calling for a change in the military's "mind set."

"I don't even know whether the successor to the M1 tank should be a tank," said the former aerospace executive, seeking new approaches to winning battles with conventional weapons in the next century.

"The first robotic vehicle that the Army put on the field was in World War I," Ambrose said. "Caterpillar {Tractor Co.} made one that was driven by field wire. It's been feasible all that time."

He said steady decreases in manpower must be accompanied by changes in tactics, such as robot infantrymen and radio-controlled armored cars and aircraft. It will take a 20-year effort to persuade Army leaders to devise ways to fight with fewer men, Ambrose predicted.

Ambrose's preference for hardware over manpower in budget conflicts with the views of many generals who have complained that the Army is too small to meet commitments worldwide and does not have the training base needed to prepare draftees to reinforce and replace volunteer troops.

The current Army of 781,000 compares with the Vietnam war peak of 1.57 million in 1968, the Korean war high of 1.59 million in 1952 and the World War II high of 5.98 million in 1945.

The proposed reduction this year to 772,600 active-duty men and women would bring the Army to its lowest strength since the 758,000 under arms in 1979. Army strength has not fallen below 600,000 since 1950 when the active force numbered 593,000.

The Air Force, Navy and Marine Corps are also cutting manpower as part of the retrenchment ordered by Defense Secretary Frank C. Carlucci to absorb $33 billion in cuts decreed in a budget summit with Congress last year.

The fiscal 1989 budget to be unveiled next week is expected to reveal only the first part of the biggest military retrenchment since Reagan took office in 1981, Defense Department officials said.

Unless Congress reverses course on defense appropriations, Reagan's successor will have to make more cuts to bring down the "bow wave" of bills for weapons ordered in the first years of the Reagan buildup, officials said.

Aggravating the problem are such unanticipated costs as the $1 million spent each day for Navy operations in the Persian Gulf and higher bills in Europe stemming from the falling value of the dollar.

Besides cutting manpower by 8,400 by Oct. 1, the Army plans to save money by halving what was to be a $60 billion program to buy a new family of helicopters, called the LHX, and trimming several other programs.

The Navy fleet is slated to be cut by 16 frigates of the Brooke and Garcia classes. The Air Force plans to shut down tactical fighter wings, one active and one reserve, and kill the "flying tomato can" antisatellite weapon, which would destroy its target through a collision in space.

For seven years, Ambrose has guided the Army through its biggest modernization since World War II.

He said the military and taxpayers would have received more bang for their bucks if, in 1981, the administration and Congress had agreed on a steady buildup costing an extra 5 percent a year rather than trying to obtain so much money during Reagan's first term.

What lies ahead for Carlucci and other administration officials, defense executives said, is a year-long wrangle within the Pentagon and in Congress over what should be cut to fit lowered budget ceilings.

Pentagon bureaucrats have noted that Army and Navy leaders are pressing already for a slice of each other's budgets and that members of Congress are taking to battle stations to protect jobs for their constituents.

Yesterday, for example, 16 House members delivered a letter to Air Force Secretary Edward C. Aldridge Jr., protesting plans to order as many as 10 days of unpaid leave for 84,000 civilians at Air Force Logistics Command centers nationwide. Another 2,970 temporary Air Force civilians would be laid off.

"Today's communique is the first step in our efforts to get the Defense Department to reevaluate its priorities," said Rep. Robert T. Matsui (D-Calif.), expressing concern about layoffs at McClellan Air Force Base in his home area.

Similar congressional protests have been generated by the Pentagon's plan to retire the 16 frigates. Lawmakers representing the ships' home ports are pressing alternative cuts in the Navy budget, including retirement of the aged aircraft carriers USS Midway and USS Coral Sea.

In a related development, Robert B. Costello, the Pentagon's new procurement chief, said yesterday that American industry, including defense contractors, wastes 20 percent to 30 percent of the cost of its products through flawed processes. He said the Pentagon could eliminate such waste with a five-year reform effort.