The software architecture based on web service has become the critical technique to construct system in the distributed environments. The web service composition is the most important method to find the correct service in the complicated application circumstance. The key question is to find service based on the QoS and how to guarantee the quality. This thesis focuses the web service composition in order to get dynamic business cooperation and integration. The key component of web service is discussed and the method of web service composition is analyzed including the formalization verification and service composition architecture and the QoS-aware composition methods. Aimed at the application of web service composition, a method based on approved genetic algorithm is put forward. The simple genetic algorithm based web service composition has many problems such as slow convergence rate and non-optimal service composition. In this paper a genetic algorithm based on niche is provided for the Qos-aware composition and it can get more accurate service composition result and can get the optimal path quickly especially in the large scale problems according to the experiment.