A modified fractal zone plate (FZP) with the character of extending the focal depth is proposed. Through calculating, the period and the structure of modified FZP with the parameter w are obtained. Within the Fresnel approximation, the diffractive field distribution and the corresponding factors of the transverse field distribution such as superresolution factor G, Strehl ratio S and sidelobe strength M have been studied using the numerical method. The properties of modified FZP in different radial coordinates (r and r2) also have been studied. The study shows that the modified FZP in r coordinate have a great improvement of focal depth than conventional FZP. The analysis of the lateral and axial properties shows that the focal depth of the modified FZP is larger than the FZP under the broadband lighting.