U.S. Senate Democratic candidate Andrew P. Miller, pressing his attack against Republican John W. Warner, yesterday accused Warner of helping develop a Strategic Arms Limitation Treaty agreement that weakened American military forces.

Warner, a former Navy secretary, immediately described Miller's attack as "astonishing" and said Miller, a former Virginia attorney general, did not understand defense issues.

"I served with three secretaries of defense and here's a man who served in peacetime in Korea with his wife living in Japan," Warner said in Martinsville.

Miller's attack on the SALT came during a day of campaigning in the Tidewater areas that has a large military population and major shipbuilding facilities as well as a large retired military population.

Both candidates have attempted to appeal to the conservation nature of the military community here and Miller made his latest effort at a news conference, focusing on the number of strategic submarines allowed by the SALT I agreement. Submarine construction has been one of ther mainstays of the nearly Newsport News Shipbuilding Dry Dock Co., which is Virginia's largest private employer.

Warner "travels across the state claiming to have 'passed upon and helped prepare' the position papers which resulted in the SALT I strategic submarine provision," Miller said.

"Those unequal submarine limitations which he takes credit for are partially responsible for the decline in our military strength relative to the Russians," he said. Miller said the agreement allowed the Soviet Union 62 submarines, but limited the U.S. to 44.

Warner has frequently cited his experience in dealing with the Soviets during a series of Navy treaty negotiations in Moscow, when he says he dealt "eyeball-to-eyeball" with the Russians.

Recalling Warner's statement during a TV debate Sept. 22 that "we never envisioned that the Russians would cheat the wat they did" on the SALT agreement, Miller accused Warner of being naive. "Mr. Warner may have been eyeball-to-eyeball with the Russians, but if so, he must have blinked," Miller said.

The attack also reflects a recent change in Miller's campaign tactics since the addit on to his staff of two new top aides, one-time campaign director James Paxton and financial adviser Tim Finchem, a Virginia Beach lawyer. Previously, Miller's campaign manager, Allen F. Clobridge, had said that the campaign would stress issues and a positive image of Miller, avoiding comment on Warner.

Talking with reporters during a stop at a Martinsville doctor's home in a room made partly of timbers from Patrick Henry's law office, Warner said he played a minor role in the 1971 and 1972 SALT I negotiations.

At the time, he said, he was heading the Navy's "incidents at Sea" negotiations, to which he often refers in his speeches. These negotiations were conducted at the same time in Moscow and were signed the day before the SALT I agreement.

Warner said the SALT I agreement must be looked at as a "composite." To take one aspect of the naval section of the agreement out of context from the sections dealing with Army of Air Force arms is potentially misleading , he said.

Warner said that if in fact the U.S. is negotiating in the current SALT II talks, from a positions of weakness, it is due to President Carter's decisions to halt construction of the B-1 bomber and other defense "concessions."

Miller said he supports the concept of an arms limitation agreement with the Soviets but criticized the SALT I pact as inadequate. As a senator Miller said he would not support SALT II unless it contains strong provisions for verifying whether both sides obey terms of the agreement.