Abstract: The deep adjustment of power supply, power grid and load structure make the disturbance behavior of the interconnected large power grid more and more complex. The profound changes of stability characteristics put forward new requirements for stability situation monitoring and emergency control technology. Firstly, this paper studied the spatial and temporal distribution characteristics of voltage amplitude and phase and the power transmission characteristics of different branches in the process of power angle swing of generators. An index of branch transient transmission capacity (BTTC) was constructed for continuous quantitative evaluation of transient stability, which has the advantages of single modulation of situation representation and criticality of judging instability. Based on the BTTC index, an emergency control strategy to reduce the risk of transient instability was proposed. The effectiveness of BTTC index and emergency control strategy is verified by simulation result got from the HVDC and ac hybrid china southern power system.