A pneumatic protection device for a triaxial air-bearing turntable and its working method are provided. The position limiting ring is located below the test platform and surrounds the air bearing hemisphere. Two first lifting support rods are connected below the position limiting ring, and each first lifting support rod is guided up and down, sliding on the linear bearing, which is fixed on the support frame. The lower part of the first lifting support rod is connected to the bearing mounting seat, and the bearing transmission shaft passes through the needle roller bearing. The two sides of the cylinder support bottom plate are connected and fixed on the support frame by connecting plate of a cylinder bottom plate. One end of the extending rod of the cylinder is connected to the cam connecting rod, and the two ends of the cam connecting rod are respectively connected to an irregular cam.