Aiming at the problem that the joint transform correlator (JTC) encryption system is difficult to perform experiments on encrypted grayscale images, a JTC encryption system based on complex amplitude modulation for grayscale images system is proposed. In this system, the complex amplitude modulation is implemented by means of single pixel hologram coding, so that the encryption of the grayscale image can be completed by using a single spatial light modulator with pure phase in the JTC encryption experiment.