Abstract: This paper propose a method of services extraction and availability evaluation of existing system when constructing a service-oriented(SOA) system.Through analysis of the relationship between the services and exchange data of Existing systems,methods and models of availability evaluation of Existing systems are proposed.Steps of this method have been demonstrated.Through mathematics approach,this paper puts forward a quantitative evaluation index for the whole availability of the existing system.Using this method,you can standardize the design process of existing system services extraction,and it has a certain judge for availability of the existing system.