Based on diffraction theory, effect of sub-aperture on point spread function of stitching system is analyzed, and after Fourier transformation of point spread function, effect on the Modulus of the OTF is acquired. The sub-aperture distribution is added to the real designed optical system through embedded program, and effect of sub-aperture distribution on image quality of stitching system is simulated. Through contrast of calculation result from diffraction theory and simulated result by design software embedded program, the simulated result is verified. Effects of sub-aperture distribution on optical systems with the same relative aperture are simulated. For reflective systems which have the same relative aperture and central-obscuration, effect of sub-aperture distribution on optical stitching system is tested. After comparision of simulated result and tested result, it is shown that, effect of sub-aperture distribution on optical stitching systems with the same relative aperture is uniform. It provides theoritical proof for practability of study on optical stitching prototype.