A method of simulating off axis electron holograms by computers is presented.In this method,the object wave on the image plane is calculated firstly,taking into account the transfer function of the image system.Then it is transmitted back to the biprism plane,on which the influence of the electron biprism on the object wave is added.By this way,the influence of the filament in the electron biprism on off axis electron hologram because of its Fresnel diffraction can be considered.Some simulated results consistent with experiments and some reference data for quantitatively analyzing the electron hologram are given.At the end,a possible approach of decreasing the influence of the Fresnel diffraction on the off axis hologram is discussed.