Important problems about the hierarchical defending of important area in post-emergency voltage control are studied. The main framework of hierarchical defending includes on-line defending in steady state and dynamic reduction in transient state. With the control validity as an index， the defending line around the important area has the character of nesting and compressibility， so a new hierarchical defending algorithm is proposed， which is based on theα decomposing way of graph theory with nesting character， to meet the need of large-scale computing in real time. The defending algorithm is applied in IEEE 39-bus example system to achieve the defending line of important area. Then with the simulation of on-line compressing process of hierarchical defense line， the validity and feasibility of protecting the power supply of important area is verified.