Abstract: Low frequency oscillation is one of the important forms threatening the safe and stable operation of the interconnected power grid. When the rotor angle of the generators swing, the network electrical quantity changes dynamically. Therefore, it is of great significance to evaluate the oscillation risks and make decision-making control based on the easily measured network disturbed responses. Firstly, aiming at the low-frequency amplitude oscillation process of the typical generator network system, the variation characteristics of the node voltage and the branch power and their relationship with the generator oscillations are analyzed. Secondly, the key branch identification is proposed to characterize the rotor angle stability of the system by reducing the dimensions. The criterion to evaluate the evolution of the low-frequency oscillation into the rotor angle instability and the emergency control strategy to reduce this risk are also put forward. The effectiveness of the key branch identification, the low-frequency oscillation risk assessment criterion and the emergency control strategy to reduce the rotor angle instability are verified by simulation in the typical systems and the AC / DC hybrid power grid.