Considering the influence of denial-of-service(DoS) attacks on electrical cyber physical systems(ECPSs), this paper studies the problem of vulnerability analysis and countermeasure synthesis for electrical network control systems under DoS attacks. Distributed control and its features are considered for the design of transmission paths of sensors and remote terminal units(RTUs) based on automatic voltage regulators(AVR) of synchronous generators. An algorithm is proposed for identifying and protecting vulnerable nodes and lines in electrical network control systems by solving a problem of optimum sparse matrix. Countermeasures of electrical communication networks under DoS attacks are further studied. The proposed algorithms are validated on the IEEE 9-node system. Spectific advice and defending strategies against cyber attacks are given according to the algorithms.