Abstract: Based on the method of wavefront phase-factor and considering of the influence of limited aperture of zone plate,we analyse the diffraction fields by Fresnel zone plate which contains three components.The corresponding integral expressions of the diffraction fields are given,the formulae of the amplitude distribution of diffraction fields alone axis are presented and a series of curves are shown.Finally,the transverse line-width and the axial line-width of that real focal-spot are obtiained.This study may further provide a theoretical foundation to optical element by Fresnel zone plate.

Key words: zone plate, wavefront phase-factor, Fresnel diffraction, Fraunhofer diffraction