The invention relates to a transient voltage stability quantitative evaluation method based on equivalent impedance of a critical system. The transient voltage stability quantitative evaluation method based on equivalent impedance of the critical system comprises the following steps of selecting a monitoring bus when a system fails; gaining an equivalent impedance curve and an equivalent potential curve of the system by using the monitoring bus and gaining a state variable and an algebra variable of a direct-current or load dynamic model at the monitoring bus during clearing of fault; establishing an equivalent system model; gaining an equivalent impedance curve of the critical system; and calculating a transient voltage stability quantitative evaluation index of the system. Dynamic response characteristics of a direct-current element, a load and the like are sufficiently considered, and transient voltage stability quantitative evaluation is realized. The transient voltage stability quantitative evaluation method can be used for on-line analysis control and off-line simulation analysis of a power system, and facilitates running of an alternating-current to direct-current system; analyzing workers can determine stability of transient voltage, identify weak nodes and timely take effective measures; and the safe and stable running level of a large power grid is improved.