An improved digital image scrambling method based on Arnold transform is proposed. The method can be used for the rectangle image by splitting rectangle image into several square images. Furthermore, a pretreatment is added to speeding up the process and enhancing the scrambling effect. The recovering of the scrambled image depends on the reverse Arnold transform that has the same cycle times with the Arnold transform. The recovering is lossless and need not calculating the period of the Arnold transform. Finally, experimental results show the robustness of the method.