For general imaging optical system, the peak position of point spread function (PSF) is located in the optical axis. Strehl ratio (SR) is defined as the ratio of on-axis values of PSF with and without aberrations, but PSF of wavefront coding system whose phase change in pupil plane is asymmetric has a shift in the image plane, so evaluation of wavefront coding system using traditional Strehl ratio is not appropriate. Shift value of PSF for cubic phase mask is carried out, which is a family of parabolas, whose focal plane is symmetrical axis. Based on this, non-axial Strehl ratio of wavefront coding system is defined, whose different parameters are analyzed, and the magnitude of Strehl ratio is inversely proportional to parameter of phase mask. Non-axial Strehl ratio is used to evaluate consistency and recoverability of PSF. Finally, an evaluation method in order to optimize phase mask parameter is given, which can determine the relation between consistency and recoverability of PSF and phase mask parameter.