To correctly reveal how voltage instability works is the key to in-depth study of power system voltage stability. The characteristics of network apparent power transmission and induction motor load are analyzed

and the transient voltage instability positive feedback dynamic process centering on the induction motor is discussed. The relationship between the induction motor load and apparent power transmission of network is defined as crossfeed characteristics between power network and the load of induction motor. Accordingly

the process of voltage collapse with the induction motor load is further accounted for. In addition

the transient voltage instability process based on the crossfeed characteristics is further clarified

and an induction motor load voltage instability criterion is proposed. A simple system model is built to verify the result. The analysis shows that the positive feedback dynamic process centering on the induction motor is an important step for the system transient voltage instability. When clearing the fault with the induction motor

the slip's corresponding restored voltage is used as a necessary condition for induction motor voltage stability judgment

improving the accuracy and applicability. A powerful theoretical support is offered by the proposed voltage stability criterion to prevent voltage instability and put forward related countermeasures.