In order to overcome the disadvantages of the double random phase encoding, a novel method for image encryption by employing the diffraction imaging technique was proposed. In this method, irisdiaphragm was used in optical diffraction encryption system, and to form amplitude plate of the different transmittance. Using this encryption system to encrypt the plaintext, multiple ciphertexts were obtained. During image decryption, by iterative phase retrieval algorithm the original plaintext is completely extracted from multiple diffraction intensity patterns. Computer simulations indicate that, since only needing to record the diffraction intensity of optical wave, the ciphertexts can be recorded without using interference methods. In addition, by means of iris-diaphragm, the transmittance of amplitude plate is easily adjusted, without changing optical structure or moving optical device, so that is greatly reducing the difficulty of implementation in the encryption process. Simulation results are presented to verify the validity of the proposed approach.