Disclosed is a gray image visual encryption method. According to the method, two processes of generating a shared image in visual cryptography and performing superimposition decoding are placed within one error diffusion feedback loop. That is, at an encoder end, each pixel or image block is sequentially processed in a particular scanning order, a superimposition decoding effect of each pixel or image block on the generated shared image is immediately checked, a difference between a reconstructed target image obtained by superposition and an original secret image is obtained by comparison, and the difference of comparison is fed back and diffused onto a surrounding unquantized pixel or image block, so as to achieve a purpose of improving quality of a secret image reconstructed by decoding; an analysis process corresponds to the process of generating the shared image from the secret image, and a synthesis process corresponds to the process of obtaining the target image from the shared image; according to the method, the synthesis is placed in the analysis, that is, the analysis is based on the synthesis.