Abstract: In this paper, the characteristics and laws of voltage amplitude, branch current and reactive power of network nodes with the increase of load power are first studied based on the typical unit-load electric power transmission for voltage stability analysis. Secondly, a combined branch location method is proposed, which combines the critical branch, quasi-critical branch and boundary branch. Based on the analysis of the error judgment caused by the variation direction of branch reactive power and different instability patterns, the paper proposes the solution to the problem and puts forward the active control strategy to mitigate the threat of transient voltage instability. Finally, the correctness of the instability criterion and the validity of the active control for the 3-machine 10-node system and the Hunan UHV AC/DC hybrid receiving terminal large power grid are verified by simulation.