Motors are main parts of dynamic loads in a power system. The characteristics of motors can reflect the transient voltage stability of a bus. However， there still lacks suitable index to properly evaluate transient voltage stability when considering mechanical torque in a induction motor load. With induction motor model and its equivalent circuit， considering the mechanical torque， an analytic method is presented to calculate the critical clearing time of transient voltage stability. The critical clearing time reveals transient voltage stability of a bus and can be used as an index. The simulation results of IEEE 30-bus system show that the critical clearing time calculated using this method can reflect the practical characteristics of motor loads and is a good index to estimate the transient voltage stability.