In recent years, with the high speed development of national economy in China, the electric power industry as the basic of economy has been developed rapidly. At the end of 2007, the total installed generation capacity exceeded 720GW which was ranked the second in the world till the end of 2002. In 2020 it will be 1360GW. The power system is composed of generating companies, grid companies and power supervising institutions mainly in China. The ¡°East-West power transmission, nationwide electricity interconnection and deregulated market¡± brings the challenges of the security, the stability and the basic research in power system in China. The blackouts reveal the hidden trouble in the security defense of power system. Then the framework of dynamic security defense in power system is constructed based on EMS and WAMS. On this platform, Static Security Assessment (SSA), Dynamic Security Assessment (DSA), Transient Security Assessment (TSA) and Emergency Security Assessment (ESA) would be realized. At last the security stability control is achieved by the key technologies of wide-area protection, control and high speed information system in the deregulated market. The major problems and the research directions of wide-area security defense system include: (1) Increasing of transmission capacity. UHV AC/DC power transmission technology, compact line, large section line, fixed series compensator, FACTS and super-conductive power transmission technology are adopted to improve the capacity of transmission lines. (2) Modeling and simulation for the loads. According to the wide-area dynamic characteristic modeling and parameter, based on the inversive simulations, the operation performance of the load model have been approached to the real physical process of practical power system to improve the accuracy and the reliability for the system security analysis. (3) The simulation theory and technique for the large-scale power system. The security analysis and the cooperation simulation for large scale power gird will achieved by the distributed calculation model and the arithmetic for the bulk interconnected network. (4) Monitoring and control for Low frequency oscillation. The power system can be controlled by open-loop suppression and closed-loop control, due to the mode of low frequency oscillations which are monitored by WAMS. (5) Transient security defense system. Emergency control is established for the transient security defense based on monitoring to the transient unstable torque-angle caused by the serious disturbances. (6) Wide-area dynamic information system. With the development of GPS and the modern high-speed digital communication networks, wide-area torque-angle measured system is constructed as a strong foundation for the dynamic security analysis from "on-line" to "real-time".