In order to satisfy the requirements of wide-area cooperation and information-sharing with different layers among some heterogeneous subsystems in power system， a multi-agent wide-area cooperation platform is proposed based on Web Service and agent technology. The cooperation model is constructed firstly; the principles of the framework implementing the platform are expatiated， as well as the features of the platform. Some key technologies， such as the design pattern of agent， the implement of Web Service components， and the negotiation mechanisms based on business event， are described. The design and implement of the prototype for wide-area backup protection illustrate that the platform can supply the dual direction communication for wide-area cooperation and greatly improve the intelligence of the system.