Defense Secretary Caspar W. Weinberger, faced with growing opposition to steeply rising military spending, said yesterday that President Reagan's planned five-year, $116 billion increase in the defense budget already has been shaved by more than a third and cannot be cut further "without risk to our national security."

Weinberger appeared to be serving notice to the new, more-heavily Democratic Congress arriving here next week, and perhaps to budget-cutters within the administration, that he intends to fight any reduction of the defense budget proposal for fiscal 1984, which goes to Capitol Hill late next month.

Although final decisions on its size are still being made, earlier forecasts indicated that he would propose that $247 billion be spent on the military next year, $30 billion higher than this year's request.

Weinberger, who has been accused by critics of allowing Pentagon spending and strategy-making to run out of control, said at a year-end news conference that "our military readiness has improved substantially" because of increases in defense spending during Reagan's first two years in office.

He said the number of active-duty units now fully or substantially combat-ready has grown by 32 percent, training time and availability of spare parts has increased, and some of the equipment maintenance backlog has been eliminated.

Weinberger also said his department is saving money through more efficient management. He said that management initiatives such as "multi-year procurement, increased competition and honest budgeting for inflation" will save $18 billion over the next five years, and that $2.3 billion already has been saved by "cracking down on waste, fraud and abuse."

But Weinberger defended his refusals to cancel $66 billion worth of new weapons systems--the M1 tank, F18 attack plane and Maverick antitank missile--that have greatly overrun cost estimates and failed at various times to perform as advertised.

"You're the prisoner of a lot of decisions that have been made many years ago," Weinberger said, arguing that it was wiser to fix weapons he said are still needed than to cancel them and start over after "that number of years and that amount of money has been sunk" into them.

A variety of government reports have shown that the M1 tank did not meet specifications, and Weinberger acknowledged that it was "indefensible and quite outrageous" for the Army to have taken 20 years to develop it. But he said "the tank, as it is now being delivered, meets the specifications that the Army requires at this point . . . is a far greater improvement" over existing tanks and a "necessary" counter to Soviet armor.

The Navy's new F18 carrier-based attack plane, which recently flunked a key test of its ability to fly the advertised 575-mile range with a full bomb load, also was defended by Weinberger as a plane that the Marine Corps wants "urgently" and the Navy "needs." He said the failures did not constitute "a final test with any of the modifications that are now being discussed," which might enable the plane to fly as far as it should.

Weinberger put the Maverick air-to-ground missile in the same category of a weapon worth modifying because it was needed.

He pointed out in a statement that Reagan submitted in March, 1981, a five-year defense spending increase of $116 billion more than the "Carter administration's inadequate defense plans" for the same period. The Reagan increase would have been even larger in impact because Carter's last defense budget, submitted after he was defeated in 1980, was higher than in previous years. Reagan boosted total defense spending to $1.6 trillion over five years.

Weinberger's statement said Reagan "proposed a $20 billion reduction in defense outlays" in September, 1981, made another $14 billion cut in 1982 because of a drop in the inflation rate, and "agreed" in May, 1982, to a further $7 billion reduction.

"These cuts total $41 billion . . . , more than one-third of the increase" that Reagan had added to the Carter plans, according to the statement.

The statement's wording suggested that all these actions had been initiated by the president, rather than being the outcome of struggles within the administration and with Congress. In his news conference, however, Weinberger referred to the total cuts as including those the president "accepted rather reluctantly from the Congress."

Weinberger's message to the new Congress, which will have 26 more Democratic members in the House, was that "the defense budget has not been exempted" from budget cuts. But he said he was "reasonably pleased" that Congress approved 96 percent of what Reagan requested the past two years, and said he was looking forward to similar support next year.

Despite his relative success in gaining financial support, Weinberger argued that "nothing can change" the basic fact of the continuing Soviet military buildup and the need to improve deterrence against attack. "To falter now would be to undo our present gains and endanger our future safety," he said.

Weinberger said he still thinks that "Dense Pack" closely spaced basing for the MX missile, rejected by Congress, is the system "which has a greater chance of enduring survivability than others we have examined." But he said the advice of a new commission being set up by Reagan to try to find an acceptable home for MX "will be eagerly sought and examined."

He also said Japan's recently announced defense budget increase of 6.5 percent "is a reasonably significant effort which we are glad to see." But he added that "this budget will not enable them to reach" the goals they have set for themselves within a certain period.