Abstract: In combination with the advantages and disadvantages of ordinary double-reflection and three-reflection, one special two-mirror optical system with three reflections is designed and the method for calculating the parameters of its initial structure is given. This initial system composed by two mirrors is a focal system, and the rays are focused after reflecting twice on primary mirror and once on second mirror. The defects of this initial system are analyzed and some steps are taken, which are beneficial to compensate the defects and obtain the better image quality. Two systems were designed with the method. The focal length of them is 5000mm, the F/# of them is 5 and the field angles are 0.4and 1respectively. The result of evaluation demonstrates that the image quality of the systems is near diffraction limit and the systems are of high resolution, long focal length and small volume.