To improve detection accuracy, Utilizing HMM (Hidden Markov model) and BW to building model, the detection accuracy improves greatly. First, the research progress of intrusion detection is recalled, then the model based on Markov and BW is presented. An example of using system call trace data which is used in intrusion detection, is given to illustrate the performance of this model. Finally, comparison of detection ability between the above detection method and others is given. It is found that the IDS based on HMM System Call sequence has improve the accuracy greatly.