Abstract: Proposed in this paper is a non-backtrace backward chaining Web services dynamic composition framework based on mediator. Under this framework, a rule modeling method in composition and a parameter-level ontology for eliminating the composition semantic conflict are provided, and then a non-backtrace backward chaining algorithm is used to compose Web services. This composition approach avoids the optimization of composition plan, and its parameter-level ontology is more efficient in handling the semantic problems in composition than the attribute-level ontology in forward chaining composition approach and is suitable for composing many times to the same user inputs and the same Web services in order to generate the same outputs or different outputs.