To realize online monitoring of system voltage stability, it is in urgent need of proposing a voltage stability index with ideal accuracy, linearity and computation speed. This paper proposes a new voltage stability index of Lu based on the feasible solution domain of branch voltage equation which can be fast calculated using the local measurement information of PMU. Moreover, in order to eliminate the error produced by redundant measurement information, the improved index Lu.pro is proposed which can be calculated merely with the voltage measurement information of PMU. Simulations are made to prove that Lu and Lu.pro are ideal voltage stability indexes with good accuracy and linearity. Furthermore, it is also demonstrated by simulations that Lu.pro can be used to evaluate the voltage stability of three-phase unbalanced system. Simulation results show that indexes of Lu and Lu.pro can indicate the weak nodes and areas of voltage stability which can be used in online monitoring and evaluation of voltage stability.