In the Fresnel diffraction field, the variation of contrast, phase and period of the grating's image are derived theoretically in the plane offseted from the Talbot self-imaging plane under the coherent illumination. The image condition of Fourier image of the grating is obtained under the illumilation of spherical wave and plane wave. So that the coincidence to some extent between the Talbot effect and Lau effect is demonstrated quantitatively. The results of theory is coincident with that of experiment.