In order to improve the resolving power of a scanning holographic system,the diffraction of a Fresnel-zone-plate (FZP) functioning as a coding aperture was studied. In consideration of the lens as a phase modulator,the integral diffraction equation of the FZP was solved digitally. As compared with the distribution of the diffraction intensity with and without a lens,an equivalent distance was introduced to simplify the analyses and a virtual Fresnel diffraction plane was proposed accordingly. Meanwhile,a critical diffraction distance was defined to describe the diffration effect of the FZP quantitatively. Experimental results indicate that the critical diffraction distance is 16 cm,when the number of fringes and the radius of the FZP are 10 and 0.75 cm respectively and the focal length of lens is 7 cm. Furthermore,the critical diffraction distance increases as the focal length of lens or number of fringes of FZP increase.These results reported here show that the complicated system can be simplified due to the introduction of the equivalent distance and the diffraction effects influenced by some factors can be quantified by proposal of the critical diffraction distance,which can provide directions for the imaging system design.