Abstract: To overcome the disadvantage of the symmetry of input and frequency planes in the previous optical image encoding systems, an improved encryption method is proposed. Instead of adding optical elements, the optical image encryption is realized by spherical wave illumination on asymmetric non-normal Fourier transform system and by placing the phase mask on the Fourier transform plane of the system. Due to the asymmetry of this system and the dependence of the Fourier spectrum plane on relevant parameters of the spotlight, it overcomes the hidden trouble resulting from the symmetry of the previous optical image encoding system and increases four fold security keys. Theoretic analysis and computer simulation indicate that this method is feasible and enhances the security greatly.