Abstract: Semantic annotation means to add semantic information to Web service based on Domain Ontology in order to make Web service be understood by the computer. So the annotation algorithm will directly affect the accuracy of Web service annotation. At present manual annotation methods are tedious, while automatic annotation methods can not guarantee its accuracy, so the research on semi-automatic semantic annotation is necessary. This paper presents a conceptual similarity algorithm, focusing on the application of KMP algorithm, which has greatly enhanced the efficiency and accuracy of annotation.