1. A service-oriented system for integrating an existing web-based application to form a service to be provided to users, the system comprises: a computer; a service inlet in the computer for receiving a service request containing an input data set submitted by a user; a service proxy interface formed by integrating the existing web-based application, for transforming the input data set in the service request submitted by the user into input parameters recognizable by-the requested service and loading a model definition document corresponding to the service according to the service request submitted by the user, the model definition document being formed by integrating the existing web-based application; and an execution engine for interacting with an external resource according to the input parameters transformed by the service proxy interface and returning the information obtained from the external resource to the service proxy interface; wherein the execution engine comprises a protocol adapter for performing service request message transformation and service response message transformation, wherein the service request message, transformation comprises mapping the requested operation and invoking for the service into a specific network resource invoking flow according to the loaded model definition document; and the service response message transformation comprises removing a protocol part related to a transmission protocol in the information obtained from the external resource, and transforming the information into a data part; wherein the execution engine further comprises a data format transformer and a data assembler, wherein the data format transformer masks inconsistency among data presentation formats, and extracts data related to the service request of the user from the data part; the data assembler assembles the data extracted by the data format transformer as a data set with a explicit business meaning according to an output message definition in the model definition document, so as to return it to the user through the service proxy interface.