An air-compressed bullet gun assembly with a length-reduced gun barrel is provided, including a barrel body, a bullet-firing device, a blocking element and a rear cover. The barrel body includes a hollow tube which further includes a 1st blocking flange and a 2nd blocking flange; the 2nd blocking flange divides the hollow tube into two connected parts: the 1st air chamber and the 2nd air chamber; the 1st air chamber includes a screw hole. The bullet firing device includes a 1st protruding flange, a 1st air-tight loop, a 2nd air-tight loop, a plurality of ventilators, and a guiding pole. The blocking element includes a locking hole, a 2nd protruding flange, and a 3rd air-tight loop. The rear cover includes a 5th air-tight loop.