High resolution, wild field of view (FOV) and high image quality are required in space and airborne remote sensing and space photography. However, the refraction system must use special materials or complex structures to eliminate the secondary spectrum, the two-mirror system possesses limited degrees of freedom in correcting aberrations, and the coaxial system has serious central shielding problem in the case of wild FOV. According to geometry optical theory and primary aberration theory, an off-axis three-mirror (TMA)system with long focal length and wide FOV was designed based on the coaxial three-mirror systems. The spectral range is visible light range, the focal length is 5000mm, the FOV is 10°, and the relative aperture is 1:12. The primary mirror and the third mirror are aspheric surfaces while the second mirror is quadratic surface. In this system, the central shielding problem is solved and the modulation transform function (MTF) is more than 0.6 at Nyquist spatial frequency 50lp/mm which is close to the diffraction limitation. Moreover, the full field diffusion plaque is controlled into 5μm. In all, the analysis results show that the image quality and each specification of the off-axis three-mirror system satisfy the application requirements.