In this paper we propose an algorithm for multiple-image encryption based on the improved virtual optical imaging (VOI). The improved algorithm enhances the security by integrating several images into one as an encrypting object and increasing key space. It focuses on three issues. First, it encrypts the phase information, which can improve the performance of decryption effect of the system. Second, it combines random phase plate with a random amplitude board at the position of the random phase plate to increase the dimension of the key space and the ability to resist the attacks. Finally, it adds a random phase plate before the image place, which enhances the security of the system. It uses the peak signal-to-noise ratio to evaluate the performance of the proposed algorithm. Compared with the traditional VOI-based encryption method, the simulations show that the proposed method demonstrates a good performance in the sense of feasibility, robustness and security.