The Marines have decided to reduce their active duty strength by 10,000 people to save money, reducing the Corps to its lowest peacetime level in 18 years.

This policy decision supplies fresh ammunition to members of Congress who say the military services need the extra billions they would get under proposals to increase the Pentagon budget by 3 percent to 5 percent annually for the next five years.

The House defense appropriations subcommittee, for example, has voted to cut the Pentagon's fiscal 1980 budget by $2.2 billion, aruging the Defense Department has plenty of money on hand, Sens. Ernest Hollings (D-S.C.) and Sam Nunn (D-Ga.) this week will try to persuade the Senate to raise the Pentagon's spending ceiling high enough to allow a 3 percent increase in the fiscal 1980 budget and a 5 percent increase in later years.

Lt. Gen. Edward J. Bronars, Marine manpower chief, said in an interview with The Washington Post yesterday that Corps leaders opted for cutting personnel as the least damaging to overall readiness to fight. The 10,000-person reduction will reduce the Corps from its currently authorized strength of 190,000 to 179,000 by Oct. 1, 1981. This would be the lowest peactime total since 1961, according to the Corps.

"The commandant realizes his program had gotten out of balance," said Bronars in explaining the results of a top-level review of how Marine dollars were being spent.

The Corps, Marine leaders said yesterday, was putting its ready-to-fight ability in jeopardy by trying to keep up its members. More vital than numbers for readiness, they decided, were buying new weaponry and repairing the physical plant that supports the Corps.

The Marines, they continued, desperately need to buy more trucks, artillery and antitank weapons, as well as refurbish repair depots and fix broken roads at their bases.

By cutting duty strength by 10,000 between now and 1981, the Corps expects to free $82 million for these and other needs. But, leaders added, this will not be nearly enough money to finance the long line of pressing needs.

One dividend from the personnel cut is less pressure on recruiters to fill vacancies in the ranks of the Marine Corps. The Corps, like the other services, is finding recruiting increasingly difficult. The Marines had hoped to sign up 47,000 new recruits this fiscal year, lowered that goal to 43,000 after the personnel cut was decided on, but will still be 1,000 persons short.

The Marine's budgeting is unique because the Corps is part of the Navy. The Navy's "blue" budget buys the Marines the planes they fly, while "green" dollars are those the commandant has to buy hardware and train the Corps.

In fiscal 1980 dollars, Corps leaders said, the "green" budget has shrunk this way over the last three years: $3.45 billion for fiscal '78, $3.36 billion for fiscal '79, and $3.22 billion for fiscal '80.