The openness of Internet offers great convenience of information sharing and exchange

accompanied with crucial challenges to Information Security.As a kind of active measure of Information Assurance

Intrusion Detection acts as the effective complement to traditional protection techniques.On the base of analyzing KMP algorithm & BM algorithm

which are fashionable pattern matching algorithms in intrusion detection system at present

the author gives an new pattern matching algorithm in place of the BM algorithm.The results show that the improved pattern matching algorithm has better efficiency and is helpful to reduce systematical missing package rate.