For the complexity of web service composition in current, this paper proposes a web service composition method based on improved DAG (Directed Acyclic Graph). This method presents the possible service matching through constructing DAG. Then it optimizes DAG by deleting the invalid composited-pathways and redundant service and adopting backward-search strategy with QoS to reduce the amount of web service. At last it calculates the QoS of choosing service to acquire the best service. In order to verify the feasibility of the above strategy, this paper makes an experiment on enterprise energy-consuming analysis service composition on interactive plat for smart power, and takes experimental analysis for services of different size. The results show that the improved strategy is more effective to generate optimizing service with high satisfaction and quality, and strategy applies to service composition on interactive plat for smart power.