In the wavefront coding researches, defocus limits and the parameters of the cubic phase plates determine thequality of the images and difficulty of restoration. We used the accurate Modulation Transfer Function (MTF) of thewavefront coding imaging system which was obtained through Wigner function to construct a novel function and discussed the relationships between defocus paramount and surface parameters. Through this method, we got the theoretical defocusing limits and the practical defocusing ranges for different systems. At the same time, by comparing the MTF function of defocus system with that of focus system, we discussed the global consistency of the defocusing MTF and tested the selection of surface parameters for different requirements of depth of focus. Finally, CODE V and mathematic calculation were used to examine the defocus limits of an optical system respectively. Meanwhile, the surface parameter for a certain system was offered.