One of the main purpose of the watermark preprocessing is to improve the robustness and security. For this reason,this paper presents an image encryption algorithm, which combines position scrambling and gray scrambling scrambled according to Arnold transform.Then all of the pixels of each sub-block are scambled by the algorithm based on Logistic chaotic map.Finally, all of the Pixels are redistributed and scrambled totally.Basing on image location scrambling,it takes advantage of multi-dimensional Arnold transformation and Logistic chaotic map, image gray scrambling is achieved. By histogram analysis,key sensitivity anslysis and correlation analysis of adjacent pixels of the results of the simulation, indicating that the scrambling effect of the algorithm is good,and the key space is large.