For the problem that the number of faulty turns is difficult to diagnose when an inter-turn short-circuit fault occurs in a permanent magnet synchronous motor (PMSM), an accurate diagnosis method for the number of faulty turns of a faulty motor offline in an offline state was proposed. Firstly, the conditions that the motor model under the inter-turn short-circuit fault of the winding and under the normal state were analyzed, the excitation source was connected to the simulation circuit, the amplitude and phase of the faulted phase current were extracted, and the fault diagnosis indicator was established by combining the excitation source value、circuit parameters and current. The simulation results illustrate that the method could accurately calculate the number of winding turns in the range from the fault-free state to the full-winding fault state. The method can be directly implemented, without installing complex equipment and additional sensors on the target motor, and has utility value in the maintenance of permanent magnet synchronous motors.