An unobscured reflective optical system with two-mirrors three-reflective is described to reduce off-axis difficulty processing. A piece of aspherical mirrors instead of the primary mirror and third mirror in the traditional off-axis three-reflective system, that is one mirror achieve two mirrors′ responsibility, the complexity of the optical processing is reduced, as well as the difficulty of the alignment. Based on the aberration theory of three reflective system, formula of the coaxial structural is deduced, the equation for the system is obtained as well. The parameters of the unobscured off-axial two-mirrors system are that focal length of 1000 mm, F number is 10, field of view is 2° × 0.4° , and the surface type of the two mirrors are all quadric surface. From the figure of spot digram, modulation transfer function, point spread function and encircled energy, it is observed that image quality is good and energy is more concentrated. The optical system is designed with small volume, simple structure, and light weight.