Based on equality of fractional Fourier transform and Fresnel diffraction and linear system theory,mathematic expressions of fractional transfer function in Fresnel diffraction system are given ,we elaborated the physical meaning and fundamental feature, and proved that routine transfer function is the special case of fractional transfer function. It is useful in fractional Fourier transform of optical measure, information optics-handling and image quality assess. The computer simulation proves the result is reliable and feasible.