Wavefront coding system with a cubic phase mask is one of important methods to extend depth of the field. This paper analyzes the characteristics of the system based on stationary phase method in space domain. By analyzing the point spread function of an arbitrary strip of the cubic phase mask, this paper points out that two stationary phase points lead to oscillations in point spread function while one stationary phase point causes a smooth curve in point spread function. Theoretical analysis illustrates that the oscillations exist in point spread function if and only if there are symmetrical components about the optical symmetrical axis. Furthermore, different areas of the point spread function correspond to different positions of symmetrical components of the mask, which is useful in the manufacture and test of the cubic phase mask. In addition, the optical symmetrical axis does not coincide with the geometrical symmetrical axis in the defocused system, causing smooth curve in defocused point spread function. As a contrast, smooth curve can not be observed in focused point spread function because of the coincidence of the symmetrical axis and the geometrical symmetrical axis.