The Polish Army, while second only to East Germany's in the Warsaw Pact, could do little to stop a Soviet invasion or to harass occupation forces afterward, U.S. military specialists said yesterday.

If the Soviets should invade Poland, they would almost certainly go in with about a 3-to-1 advantage, officials said. And they would come in from several directions at once, the specialist predicted, making an effective defense by the Polish army virtually impossible.

Also, these officials said, the Soviets have supplied Poland with tanks and planes too old to be a match for the Russians' own. Unlike the situation in Afghanistan, there is virtually no chance of an outside country getting additional arms into Poland.

The officials said the Soviets currently have two divisions in Poland ready to march -- one in the northeast near Starogard and the other in the southwest near Swidnica. These are fast-moving motorized divisions of about 10,000 to 12,000 troops each.

The Soviet high command has put divisions on alert in the Soviet Union, East Germany and Czechoslovakia, from which armored divisions could drive quickly into Poland by road.

Specialists said there are five divisions in Czechoslovakia within easy striking distance of Poland, and three divisions on full alert in the Soviet Union just across Poland's eastern border. These divisions include reservists who have been ordered to active duty, disrupting the economic life in Soviet villages nearest Poland.

Against Soviet forces that could be expanded to 300,000 troops in a few days and more than that if needed, Poland has an army of about 210,000 men -- 154,000 of them draftees in for two years. Most of the Poles' weaponry is at least a generation behind what the Soviet troops would be likely to bring. Polish tanks, for instance, are the old T54 and T55, which would be no match for the Soviet T72.

In addition, the Poles' Soviet-suppled antitank missiles are the Snapper and the Sagger, which did well for the Egyptians in the Battle of the Sinai in 1973 with Israel but are not suitable for the twisting roads of Poland, The U.S. specialists said. The antitank missiles rise sharply after being fired, and the only start getting on track after flying about 800 meters.

The Polish and Soviet air forces are similarly mismatched.The former has 85,000 men with 700 combat aircraft, featuring the old Mig21 fighter, which would be blown out of the sky by more modern Soviet warplanes.

One of the hardest tasks facing the Poles, in the view of the U.S. U.S. specialists, would be coordinating their defenses, which would require elaborate preparations involving three headquarters.

Government sources said the Soviets have kept the Polish army low on ammunition and have recently increased the number of Russian "advisers" sent to work with Polish divisions. The Pentagon's view is that the Soviets are taking every precaution to prevent the Poles from putting up any kind of coordinated opposition.

As for the possibility of Polish forces retreating to the hills and harassing the invaders like the Afghans have done, specialists noted there is little cover for guerrilla forces in Poland, no sanctuary or source of supply across the border, and no civilian population likely to risk helping guerrillas. "The attitude probably would be passive resistance," one official said.