Abstract: Active short circuit (ASC) is often used in the permanent magnet synchronous motor (PMSM) drive system of electric vehicles to avoid the damage of power battery caused by high counter electromotive force of the motor under high speed driving conditions. This method has the disadvantage of the large peak value of transient current on the AC side, which can easily cause overcurrent damage of power devices. Linear variable modulation ratio active short circuit (LVM-ASC) can effectively reduce the transient current peak, but no mechanism model has been proposed. In this paper, the maximum peak current of the motor under the ASC method was quantitatively analyzed, the discrete current model of the system under the LVM-ASC method was established by piecewise linearization, and a design method of variable modulation ratio time parameter was given, which can improve the flexibility of the active short-circuit protection strategy. Finally, the effectiveness of the model and parameter design method has been verified by simulation and experiment.