In order to improve the encryption algorithm’s resistance ability to differential attacks, the paper proposes a modified image encryption algorithm based on confusion and diffusion. First original image is divided into several blocks, and then each pixel in the block is executed confusion substitution based on bitwise operation, each sub-block pixel was fully diffused to other sub-blocks in turn to weaken the image correlation between adjacent pixels effectively. Finally, using the pixel diffusion operation can effectively improve the algorithm's ability to resist known plaintext attack. The experimental results show this algorithm not only has highly sensitive key, but also has higher scrambling degree and stronger resistance ability to differential attack than uniform scrambling and two-dimensional chaotic system of image encryption.