With the advantage of its simple structure and good image quality, the pan-Cassegrain is widely used in the field of space optics. Based on the modulation transfer function (MTF) measurement results of the pan-Cassegrain optical lens with the spectrum band of 500~800 nm, and combined with the characteristics of pan-Cassegrain optical system, the cause of chromatic aberration is analyzed, resulting from its high-order aspheric, and a new method to improve the perfermance the optical system is proposed correspondingly based on the minimal chromatic aberration of aspheric. According to the third class optical aberration theory, the surface equation of Schmidt aspheric is analyzed and the minimal chromatic aberration and its zone-aperture height are figured out. Improvement of optical system is achieved correspondingly and the results show that the full-aperture spherochromatic aberration of improved optical system is reduced from 31.4 μm to 4.25 μm, and its MTF values are more than 0.6. The results provide a good guiding to the design of other similarity optical systems.