High-speed aircrafts which use splicing optical windows can reduce the drag and radar reflectivity. But they will cause the incident wave at the front of the various window glass splited, and may cause the degradation of the optical system modulation transfer function (MTF) and the decline and splitting of the point spread function (PSF). In severe cases it will affect the resolution of the system. Therefore, analysis of the effect of stitched windows on the optical systems has an important reference value to the optical system design using stitched windows. Use Zemax software to build stitched window model, we simplify the process of the analysis. The results show that when the incident light is divided to produce a specific optical path difference, the MTF and PSF will decline and split. The serious split of the PSF will decrease the image quality of the optical system. Reasonable choice to the stitched window parameter can mitigate this effect.