Abstract: In order to overcome the known plaintext attack which the asymmetric optical image cryptosystem based on phase-truncated Fourier transforms can not resist, an encryption method based on phase-truncated Fourier transforms was proposed by employing the phase factor of the spherical wave under the spherical wave illumination. The theoretical analysis and experiment results indicate that the proposed encryption method can maintain the asymmetric characteristic of the cryptosystem and avoid various types of the currently existing attacks, especially the known plaintext attack, which the asymmetric cryptosystem based on phase-truncated Fourier transforms can not resist. The results are helpful for the security improvement of the asymmetric optical image cryptosystem based on phase-truncated Fourier transform.