Abstract: The Lorenz chaos system was applied to the digital image encryption based on the thought of the higher secrecy of a three-dimension chaos system. Firstly, it carried out pretreatment to a real chaos sequence produced by the system. Secondly, the index matrix of pixel permutation was structured directly by this real chaos sequence. Finally, digital image encryption was realized in the space domain. The results of analysis and simulation show that pretreating the real chaos sequence has the stronger pseudo-random, the better autocorrelation and mutual correlation characteristic. Additionally, the bigger secret-key space of the three-dimension chaos system, the stronger the algorithm can be against decryption attack. Three outputs of the system not only can realize the parallel encryption of three or more images, but also increase the more encryption efficiency of the algorithm.