This paper analyses the theory of KMP algorithm, using the irrelevant mismatching feature of main string and substring, proposed a kind of duplex layer structure hierarchical nesting string matching algorithm, and apply the improved algorithm to flow filtering technology. Under the condition of "large text - big Mode" string matching, the improved algorithm solves the problem of strong dynamic memory share and time-consuming about calculating the next value in the KMP algorithm. The experimental results suggest that the algorithm has higher matching efficiency under the condition of large data sets matching.