The size of a single CCD cannot meet the requirements of modern optical system. To enlarge the field of optical system, a reflector based butting system was adopted. Aiming at the problems of vignetting and decline of modulation transfer function caused by butting, a reflector based butting system which has 6 reflectors was investigated. By analyzing the feature of vignetting, a sub-image pixel overlapping method was proposed to reduce the vignetting. At the same time, vignetting at edge was restored, and the modulation transfer function was improved by compositive point spread function. The results show that though the cut-off frequency has not been increased, the value of modulation transfer function is improved obviously.