Abstract: To generate digital chaotic sequence and apply that in hardware encryption system, this paper analyzes Chaos’complexity quantitative analysis methods and presents the approximate entropy as criterion of measuring the complexity of the chaotic sequences. Set tent and logistic two kinds of chaotic systems as examples, analysis and comparison their complexity.It is proved that the method can distinguish different complex chaos and chaotic sequences on the basis of short observed sequence for judging the criterion mentioned above. Researches show that the complexity of the Logistic map is better than Tent’s. The results of the study provide the theoretical and experimental basis for the application of chaotic sequence in the information security communication.