The development and applications of wide-area measurement system provide great potential for online assessment of voltage stability in power system. The study of voltage stability is considered as new challenge based on dynamic measurement. Power system voltage instability mechanism is explained based on character of P-V curve and V-Q curve, and the criterion is given that be able to estimate voltage instability by following dynamic measurement. The criterion can be used to estimate voltage instability according to the trend of change of current, power, active power and reactive power after disturbance. Moreover, the criterion has the characters of clearness in principle and convenience in application. Efficiency and correctness of analysis methods and criterion of voltage instability that proposed in this paper are verified with numerical simulation examples under different conditions.