The Joint Chiefs of Staff have warned civilian superiors that the recent 7 percent pay increase will not be enough to stem the exodus of essential people from the armed services.

The chiefs, in a memorandum to Defense Secretary Harold Brown last week, said they had concluded that disillusionment with military pay is the biggest reason why the Army, Navy, Air Force and Marine Corps cannot keep key skilled enlisted people and junior officers in today's all-volunteer military.

The choice for civilian policymakers, the chief wrote, is to pay high enough salaraies to retain skilled people or settle for a military less ready to fight in the future.

The chiefs, though plainly indicating the 7 percent pay raise will not be enough, did not specify percentage increases for the future. They did stress, however, that military salaries must be raised enough to compensate for inflation.

The Pentagon already has sent to the White House a proposal to add $650 million to the fiscal 1981 budget to finance such "sweeteners" as larger housing allowances and extra pay for enlisted people and junior officers with key skills.

As with the 7 percent general pay increase, the chiefs wrote in their memo that the $650 million package is a good start but not enough.

Their plea for higher pay to retain such key people as enlisted electricians and aircraft mechanics represents the latest round in the debate over how much is enough for national defense.

President Carter is preparing a fiscal 1981 budget that is expected to call for $144 billion for national defense, compared to $130 billion this fiscal year.

In hopes of persuading senators that the pending strategic arms limitation treaty (Salt ii) is an acceptable risk for the United States, Carter has promised to give Congress an advance peek at his future military spending plans.

The preview of the fiscal 1981 budget is expected to be given to Congress before the Thanksgiving recess.

Secretary Brown and Pentagon Comptroller Fred Wacker went over the fiscal 1981 defense budget with White House budget executives yesterday.

The Carter administration is committed to increasing the defense budget by at least 3 percent a year after allowing for inflation. Some lawmakers are insisting on 5 percent.

A 3 percent increase on top of the current annual inflation rate of 13 precent would push the $130 billion military spending estimate for fiscal 1980 up to $150.8 billion. A 5 percent real growth, on top of 13 percentt inflation, would further increase the $130 billion total to $153.4 billion.

The administration hopes to reduce the rate of inflation, which would lower those totals.

Despite the prospect of extra billions coming the Pentagon's way next year, Navy leaders are complaining that their slice of the money pie will be smaller and the Air Force's larger in fiscal 1981.

The main reason is that about $2 billion is expected to be earmaked for the Air Force's MX mobile missile. The Navy, as the budget debate stands now, will have only enough money in fiscal 1981 to buy about seven or eight large warships and submarines.

Army and Marine Corps leaders, in lobbying for money in the fiscal 1981 budget, are stressing the need for responding to a distant emergency, The Army wants to keep building its arsenal stored in Europe while the Marines are enthusiastic about ships converted into floating warehouses and berthed near potential hotspots.