Abstract: Rotor angle stability and voltage stability are two different stability modes in electromechanical transient scale of ac and HVDC hybrid power grid. Fast and accurate identification of dominant mode is the premise and foundation of effective stability control. Firstly, based on two typical systems, this paper analyzed the temporal and spatial variation of node voltage amplitude and phase under different dominant stability modes, as well as the difference of response characteristics of different AC branch sections; secondly, in order to identify the critical branches that can characterize the system, a quantitative evaluation index of simplified branch transient transmission capacity (sBTTC) based on response information was defined. According to the position of critical branch's pedal voltage, the method to distinguish the dominant stability mode and the emergency control strategy to reduce the risk of system instability were proposed. For AC and HVDC hybrid power grid, simulation results verify the effectiveness of the identification method and emergency control strategy.