Abstract: With the continuous expansion of the AC/DC hybrid power grid scale, the stability characteristics become more complex, and rotor angle instability and voltage instability are often interweaved and difficult to identify. A fast and accurate methodology that identifies the dominant mode of coupling instability is the priority of targeted emergency control strategy. In this paper, temporal and spatial variation characteristics of branch reactive power is analyzed based on three typical systems, and differentiating features between temporal and spatial variation characteristics are extracted. Then, combined with the reactive power product and the voltage product at both ends of the branch, the identification indicator with fixed critical value and clear physical significance is proposed to indicate the dominant mode of coupling instability as well as to determine the position of vertical voltage drop point. Finally, voltage product is utilized to identify the critical branch, and a discrimination method of the dominant mode and the emergency control strategy are proposed. The proposed method and strategy are validated based on the standard system and the AC/DC hybrid power grid.