By studying the optical transfer function (OTF) of wavefront coding (WFC) imaging system with cubic phase mask, we find that modulation transfer function (MTF) of WFC system is defocus-independent under a special condition. Compared with the traditional optical system, a formula of extension ratio of the depth of field (DoF) for WFC imaging system is obtained. Through analyzing the range of spatial frequency and the coefficient of phase term α, the conclusion is drawn that the DoF of WFC imaging system can be extended more than 60 times when α＝20π.