Because of the flexibility of Web Services and dynamical network, QoS is difficult to be assured of reliability, often causing the Web Service selected and invoked by users are often not working properly, not result in high performance Web service composition. In the composition of services for service selection, in order to improve the reliability and performance of composite services need to consider the services of non-functional factors, the paper apply of knowledge of probability and statistics predicting Web service's dynamic QoS property values, propose a objective evaluation of the credibility of Web Service and a improved K-MCSP QoS global optimization algorithm of service composition for improving the reliability of service composition.