Abstract: The secrecy of encryption image usually decreases when using low dimension chaotic sequences. A bitmap encryption algorithm was proposed based on the chaotic sequence, and a composite sequence was introduced. Through analyzing the dynamics of Logistic maping chaos system, three propositions were presented to improve the method for producing chaotic sequence. A composite sequence is composed of this chaotic sequence and Henon sequence, its security is enchanced because the distribution functions of chaotic subsequences are concealed by the composite sequence. Encryption was implemented twice when using two different techniques of image scrambling transformation and substitution in the space and DWT domain. Experimental results and theoretical analysis show that cipher image completely depends on secret key, and can resist the attack of common attack algorithms.