The diffraction self-imaging phenomenon of the grating in the optical system-general Talbot effect, is studied with aid of the Fresnel-Kirchhoff integral in the Fresnel approximation. The geometrical imaging is considered as a special case of Talbot imaging. According to Talbot formula, the properties of the diffraction self-imaging of the grating illuminated by parallel and spherical wave are discussed respectively. Finally, Talbot effect is explained by the method of the interference.