The diffraction of Fresnel-zone-plate (FZP) functioned as coded aperture is studied through numerical calculation and experimental measurement. Considering the lens as phase modulator

the equivalent distance at which the same distribution of diffraction intensity is generated with lens is obtained by comparing that without lens. The virtual Fresnel diffraction plane is introduced accordingly. In order to analyze the diffraction of FZP quantitatively

a critical diffraction distance is thus defined. The influences of focal length of lens

fringe number and radius of FZP on diffraction are measured in experiments

which testifies the calculation. Moreover

the diffraction with incidence of spherical wave is discussed. In the end

some suggestions on the design of FZP

the focal length of lens and the distance between an object and the lens are proposed.