For demands of optical systems in a off-axis three-mirror camera on the optical-mechanical structure, an appropriate optical-mechanical structure was designed by taking Carbon Fiber Composite(CFC) as the key components of the camera. The weight of CFC in designed camera makes up 32% that of the total system. Furthermore, this optical-mechanical structure is asymmetric, the system spacing in the primary, secondary mirror is 850 mm, and the mirror positioning accuracy demands its interval, eccentric and tilt to be 0.005 mm, 0.005 mm, and 5″, respectively. The Finite Element method was used to analyze and optimize the design, results show that the optical-mechanical structure has better stiffness, lighter weight, and can meet the requirements of optical system for intervals, eccentrics and tilts. A mechanics environmental test and a thermal vacuum test were performed, and results demonstrate that the optical-mechanical structure in the camera has excellent stability in mechanics, thermal and other environmental conditions, its first order harmonic frequencies are above 120 Hz, and the modulation transfer function is more than 0.2. These results prove that the proposed structure can provide reasonable spatial location and stability for the reflective mirrors in off-axis three-mirror cameras.