To meet the relative position request of the second mirror and the primary mirror in a large-aperture and long-focal-length space camera, a combined supporting structure between the primary mirror and the second mirror was designed and manufactured,and the stability of the combined supporting structure was confirmed through the analysis and tests. Firstly, according to the optical system，the combined supporting structure which combined a thin wall joint cylinder and a supporting bar was designed. The structure forms of supporting bar and joint cylinder were compared and their connection modes were discussed. Then，the gravity response，characteristic frequency and sinusoidal vibration responses of the supporting structure were analyzed. Finally, the supporting structure stability was validated by the mechanical tests and optical measurement. The results indicate that the angle displacement between the primary mirror and the second mirror is less than 10″ and the first-order natural frequency of supporting structure is greater than 75 Hz,which shows that the supporting structure has a better structure stability, and can satisfy the usage requirements of space cameras.