The Reagan administration yesterday opened a $55,000 show-and-tell in Congress' backyard, giving lawmakers tours through one of the Airborne Warning and Control System (AWACS) planes President Reagan wants to sell to Saudi Arabia.

Air Force briefing officers, with the help of crewmen sitting at consoles aboard the AWACS parked at nearby Andrews Air Force Base, told their visitors that the militarized Boeing 707 with the giant radar donut on top was a defensive weapon, not an offensive one that would threaten Israel.

Two of the briefing officers, asked by reporters whether they felt they were being politicized by the administration, answered no on grounds that AWACS would be a boon to the American as well as the Saudi military by keeping an unblinking eye on the unpredictable Middle East.

The AWACS that was flown into Andrews on Sunday and will be kept on display through Wednesday at what the Air Forces estimates will be a cost of $55,000 has already patrolled the skies over Saudi Arabia with Israel's blessing. Americans--not Saudis--were at the flight controls and tracking radarscopes, however.

But under the proposed new arrangement, which Israel opposes and the administration is having trouble moving through a skeptical Congress, the United States would sell five AWACS to the Saudis, maintain the planes for three years, build associated ground equipment and teach the Saudis how to fly the plane and operate its sophisticated electronic gadgetry. The Saudis would pay $5.8 billion for all this.

The Air Force trained its fire yesterday on the arguments that the Saudis could use the AWACS to attack Israel and that selling the planes to Riyadh increased the risk of the highly secret electronic gadgetry in the plane falling into Soviet hands.

"The only offensive role it would play," said Air Force Maj. Gen. John L. Piotrowksi of the AWACS in briefing reporters at Andrews, "would be to provide warning of aircraft that had gone beyond their political borders" and into Saudi air space.

The AWACS sends radar beams down from its position six miles above the earth to detect invading aircraft which, by flying close the earth, can evade ground-based radar until it is too late to scramble fighters. But, Piotrowski stressed, these same AWACS beams are not discriminating enough to pick out tanks so ground forces could destroy them in a land war, a capability that would worry Israel.

As Saudi defenses stand, the general continued, enemy fighter-bombers probably could destroy the American-supplied Hawk anti-aircraft missiles before they could be fired. Saudi radar probably would not detect the invaders in time to scramble its fighters, either. AWACS would increase the warning time from about three to 11 minutes, Piotrowski said.

He acknowledged that AWACS could serve as an airborne controller in wartime, directing fighters toward other nation's planes. But he said this is best done with the AWACS orbiting well behind its front line of fighters, not near a hostile border where enemy planes could shoot it down or jam communications with ease.

Allan Schell, a civilian electronics executive working for the Air Force's Rome Air Development Center, and Lt. Col Richard F. Stamm of the Air Force staff at the Pentagon, said in a separate briefing that most of the equipment aboard the AWACS could be bought commercially.

Therefore, they said, the big intelligence payoff for the Soviets if they captured an AWACS would be in its operating characteristics, not the computers, radar and tracking hardware developed in the 1960s and early 1970s.