The ray tracing method in geometric optics is applied to acquire the asymptotic expressions of ray aberrations for phase plates of free form illuminated by collimated beams with large field of view, with which the spot diagram characteristics of the wavefront coding imaging system with anti-symmetric phase plates, including spot size, boundaries, and ray intersection structures, and ray aberrations enhancements and spot shape variations due to the large view-field angle are analyzed. The corresponding asymptotic expressions to describe the above characteristics are presented. With this spot diagram characteristics analysis, a deeper insight into the wavefront coding imaging systems is acquired and instructions for designing real systems are found.