The digital image encryption algorithm based on chaotic system has become a hot research topic in the domain of image encryption. This paper proposes an image encryption algorithm based on improved logistic map and Block-Mod algorithm. Firstly, a chaotic sequence is generated by improved logistic map, scrambling the positions of the image pixels in the spatial-domain. Secondly, the pixel values are diffused using two rounds Block-Mod operation. Finally, the pixel position scrambling is performed again, the encrypted image is obtained. Through Matlab simulation experiment, the information entropy of image encryption reached 7.999, the adjacent correlation coefficient of the three directions are within ± 0.1, Experimental results show that the algorithm has a large key space, high key sensitivity, can effectively resist statistical analysis attacks, gray value analysis attacks and other attacks, with good encryption effect.