Many methods have been used such as UDDI and DWS to discovery requested web services. But they are just a kind of simple syntax match based on keywords and have a low ratio and precision. This paper proposes A Semantic Web Service Discovery Method Based on Ontology. It uses tree-form data structure to describe the web services and give all the nodes a weight value by certain strategy, then compute the semantic similarity between the web services requested and the services registered. To validate the feasibility and effectiveness of the algorithm, we construct a self-developed prototype system to show how well it works. The experiments prove that this algorithm has high recall and precision than other methods.