A position limiting protection device for a test platform of a triaxial air-bearing turntable and its working method are provided. The triaxial air-bearing turntable is installed on a support frame and includes an air bearing support seat and an air bearing hemisphere. A test platform is placed on an adapter plate, which is placed above the air bearing hemisphere. The position limiting protection device includes three top rods and three annular parts, which are sequentially spaced to form a position limiting ring, the position limiting ring is located below the adapter plate and surrounds the air bearing hemisphere. The position limiting ring is spaced at a certain distance from the air bearing hemisphere, each annular part is fixed on a support frame, and the top rods are controlled to rise or fall to limit the movement of the test platform.