An approach based on backward tree to compose services automatically is proposed. It composes services for a user through three steps: 1) Builds a complete backward trees on-line; 2) Searches for optimal valid generation sources (generation-paths); 3) Composes generation paths. Compared to traditional graph-based methods, it has a smaller search space and avoids the repetition search. Experimental results show that this method has a good performance even the repository has a large number of services.