When the negative sequence current is taken as the fault characteristic quantity of interturn faults in stator windings of asynchronous motor, its extraction is easily affected by harmonics and noises and the spectrum analysis is rather complicated, so it is difficult to balance the reliability and efficiency of fault diagnosis. In order to diagnose interturn faults in stator windings of asynchronous motor reliably and efficiently, on the basis of multiple Park’s transformations, the fault characteristic quantity Izpg was put forward to reflect the fault. The current frequency in the stator after the fault was deduced according to the motor magnetic field after the fault firstly. Then the expression of fault characteristic quantity Izpg extracted by multiple Park’s transformations was obtained under nonideal working conditions. Finally, the multiloop mathematical model of asynchronous motor after fault was established, and the interturn fault of stator with different severity under ideal and nonideal conditions was simulated. The analysis results showed that the Izpg was positively correlated with the severity of the fault and had robustness to nonideal conditions such as voltage source imbalance, and it was highly sensitive and reliable to diagnose the stator interturn fault of asynchronous motor by Izpg.