In order to solve the problems faced by the security of interconnected power grids in China, the principle and method of power fluctuation and the oscillation of tie line are put forward, and a multi-time-scale dynamic whole-process model and its simulation method of power system in China are developed. Base on them, the technical system of the dynamic process security defense of interconnected power grids is established, which can improve the effective defense against interruption of tie line chain and accurately simulate the dynamic process of tens of minutes or even hours. It provides a technical guarantee for the safe and stable operation of China power grid, which reaches the highest level in the world.