Based on three-order induction motor load model and considering different mechanical torque, a method for calculating critical clearing time under rapid assessment is presented. The effect on critical clearing time by rotor winding electromagnetic transient characteristics and mechanical torque parameters is analyzed. The proposed method improves the accuracy of induction motor load model, and the time index can be calculated more simply so that this method can be used in dynamic security assessment system as a computational engine. The simulation result of IEEE30 bus system shows that the power of mechanical torque benefits the transient voltage stability of load nodes, and the electromagnetic transients in rotor winding harm the transiet voltage stability; the effect of model accuracy on critical clearing time is smaller than that of mechanical torque parameters.