The invention discloses a web service composition method based on an improved QPSO algorithm. Based on the Web service task numbers, each task has ni (i=1, 2, 3.....m) candidate Web services having same service functions and different QoSs, the Web services are encoded at a fixed length based on the task numbers, the number of the services is shown in the description, services meeting user demands and constrained conditions can be found, and service instance numbers corresponding to particles are resolved based on the improved QPSO algorithm, namely, an optimum service sequence can be obtained, and a larger service function can be provided for the Web service composition. The iteration frequency for achieving an optimal solution is reduced and the practical significance for building complex composition services based on Internet is achieved.