To solve the problems in conventional design of three-mirror system such as complicated initial structure solution and slow convergence when using aspheric surfaces, a design for off-axis three-mirror optical system based on Wassermann-Wolf differential equations is proposed. According to the law of reflection and sine condition, the Wassermann-Wolf differential equations for solving coaxial three-mirror initial structure are derived. The coaxial three-mirror initial structure with good image quality is obtained by fitting Wassermann-Wolf surfaces with polynomial. An off-axis three-mirror optical system is achieved with long effective focal length (1200 mm), wide field of view(18°×4°), and large relative aperture (F number is 4). The modulation transfer function is over 0.5 at 50 lp/mm in every field of the system. Result of the design indicates that this method based on Wassermann-Wolf differential equations is simple, effective, fast in convergence, and it provides a good starting point for the design of three-mirror optical system. Besides, the primary mirror is of conic surface, and both the second and tertiary mirrors are aspheric. The three reflectors are neither eccentric nor tilted, so the fabrication cost as well as the alignment difficulty are effectively reduced.