Traditional strategy of designing a two-mirror catadioptric optical system employs zero-power correcting elements to balance the residual aberrations, which usually contains two or three refracted, even asphere, lenses. The use of an axial gradient-index (AGRIN) lens can achieve the same goal of aberration correction, as well as a compact and light weight system which employs only one correction element. This paper summarizes primary aberrations theory of the two-mirror catadioptric optical system and the AGRIN lens. A method of designing a twomirror catadioptric system with an AGRIN lens instead of homogeneous lenses is presented. As an example, a twomirror catadioptricsystem, whose focal length is 3000 mm, F-number is 8, and the field of view is 1°, is promoted. The design result shows that the wavefront aberration is less than 0.07 l, the system dispersion radius is close to the radius of Airy, the transfer function at 50 lp/mm is higher than 0.35, the maximum distortion of the system is less than 0.5, the maximum field is less than 0.2, and the image quality almost reaches the diffraction limit.