The composite Web services provide users with a more efficient solution by the components provided by third parties. The selection of the appropriate components among a lot of available Web service components often rely on the QoS of the components. After the system model has been established, we need to calculate the aggregate QoS according to the QoS of each component, which is a heavy workload. This paper presents a method to calculate the aggregate QoS automatically according to the connection relation of the system model. The method that provides an automated tool for the quality assessment of the composite services is versatile and capable of calculating according to different definitions of the aggregate QoS.