Applying the wavefront coding technique to optical system can greatly increase the depth of focus and correct the focus-related aberrations. While most coventional imaging characteristics analysis of the wavefront coding system is carried out within spatial-frequency domain, here the analysis within spatial domain was applied. The approximate expression of point spread function for the wavefront coding system with cubic phase mask was derived with the stationary phase method, based on which the sensitivities of the wavefront coding system to defocus, astigmatism and coma were analyzed, and the boundary, bandwidth and oscillations of the point spread function were described. A further insight into the imaging characteristics of the wavefront coding system was acquired within spatial domain.