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Chen G et al. presented a 3D chaotic cat map based symmetric image encryption method and Kai W et al. broke it only with the knowledge of symbolic dynamics and some specially designed plain-images. In this letter we point out why the original scheme was vulnerable to the proposed attacks. Some essential weaknesses in the original scheme are discussed here. Based on the analysis, an improved encryption scheme with multi-chaotic map is presented to achieve higher security. In our proposed method, the 3D cat map with random scan processing is developed to increase the confusion process; the combinations of coupled map lattice (CML) model, tent map and logistic chaotic map are employed in the diffusion process to improve the initial sensitivity and security of the encryption system. In addition, entropy is used to evaluate the performance of the proposed encryption system. Experimental results show that the presented method can resist various kinds of attacks and provide an effective way for high security image encryption.