An approach to automating Web service composition is proposed in this paper. Firstly, the required service is regarded as a process-based workflow created according to the client requirement and represented with extended deterministic finite automata (EDFA). Secondly, the approach searches useful atomic processes based on the parameters processed by the required services. Finally, a conditional tableau algorithm of Description Logic is used to compose these atomic processes. The input provided by the client and these atomic processes found in the previous step are represented with description logic formulas; if there is a model that satisfies the conjunction of these formulas, this model is transformed into an EDFA to extract the composite Web service with the semantics-based intersection of EDFAs. This approach sufficiently employs the existing web services and accurately extracts the composite Web service satisfying the client requirement.