A novel optical encryption system based on non coherent superposition is proposed. In this system, the original image is hidden into three Phase Only Masks (POMs). One of the phase masks is generated by computer while another two are obtained analytically. For decryption, the diffraction filed of the three POMs is superposed by utilizing the beam splitters. The intensity of the superposition field, which is exactly the original image, could be recorded by CCD camera. Compared with the earlier interference-based method, our approach is simple and easy to be realized by optical elements. Moreover, the silhouette problem existing in the earlier method with two POMs can be resolved in our method, thus the security of the method is further improved. Simulation results are presented to verify the validity of the proposed approach.