Brsed on the wave theory of optics, the affect caused by the phase error in Fresnel near-field diffraction equation is analysed. Taking single slit as an example, the relationship between phase error and resulting accuracy is given. Prom calculating of the focal plane intensity distribution on Linear Presnel Zone Plate, the results of using Fresnel diffraction equation is stated when the phase error requirement is not satisfied. Some facts affecting the focal plane intensity distribution of LFZP are given.