A novel formal representation of agent organization is proposed which gives a theoretical basis for multi-Agent system. Then， based on the requirement analysis of wide-area backup protection（WABP）， a WABP algorithm is given. This algorithm uses the expert system combined with Bayesian network for fault diagnosis. Three architectures for the WABP system are compared and analyzed， and as the result， the regional integrated architecture is chosen. The organization structure of the WABP system is studied. The designs of core regional decision Agent are given， including the working process， the uniform interfaces and its relation graph of member-role-capability. The test results illustrate the rationality of the model， and provide a reference for the engineering development of WABP systems.