The basic concept of service mapping repository, collaboration network and service task circumstance was introduced. An approach based on collaboration network and task circumstance to compose services automatically was proposed. First, this method built a complete collaboration network for every output; next, the collaboration networks were combined into a composite service to meet the needs of users; finally, best composite service was returned to the user based on compatibility of synthetical task circumstance. The experimental results show that the method proposed can improve the success rate and efficiency of service composition.