Based on the geometrical optics theory of three-reflection optical system, the design method of the system is discussed in detail. According to primary aberration theory, a calculation formula of the three-reflection system initial structure is deduced. And the relationships among system structure, basic structure parameters and focal length are obtained. The characteristics and the applications of three-reflection optical systems are discussed through design examples. The optical system with wide field of view, small F number and diffraction limit imaging is designed to suppress stray light effectively.