The definition, the role, the constructing modes, and the key technologies of wide-area protection system for power grid are introduced. A new scheme of wide-area protection system is proposed. The proposed wide-area protection system is based on synchronized Multi-Agent System (MAS) self-organizing online. In order to improve the real-time performance, all of the agents in the proposed system are synchronized by s set of precise synchronization pulses. According to requirements and setting rules, specific function subsystems based on MAS can be constructed by self-organizing mode online. Therefore, it is flexible to construct subsystems for various purposes. The proposed system is of the capacity to achieve the functions of wide-area backup protection and to coordinate the main protections and the control equipments for enhancing stability of power system. The proposed scheme is introduced in detail, and the cooperation problem with traditional protection devices and control equipments for improving power system stability and security is discussed.