A new way to on-line voltage stability monitoring has been provided with the combination of phasor measurement unit（PMU） measurements and Thevenin equivalent model. The disadvantages of traditional Thevenin equivalent model are analyzed qualitatively based on circuit analysis theory. An improved Thevenin equivalent model is proposed based on the analysis. Measurement equations， solvability conditions， solution methods and error analysis of the improved model are studied. The improved model is applied to on-line voltage stability monitoring， with load margin as a voltage stability index. Furthermore， several aspects concerning the proposed model are studied including data window selection， model degeneration， and application environment. The IEEE-39 bus system is used as a testing system. Simulation results demonstrate better precision and adaptability with the improved model.