Aiming at the problem that the flux linkage signal is unstable and difficult to diagnose when the permanent magnet of permanent magnet synchronous motor (PMSM) occurs the loss of field fault, an on-line diagnosis method of PMSM demagnetization fault based on extended Kalman filter（EKF） was proposed. The EKF method that could identify on-line and dynamically correct the flux linkage was constructed. This method can track the real state of the permanent magnet flux linkage signal according to the adaptive PMSM model, and it is not easily affected by the motor parameters. At the same time, the identified permanent magnet flux linkage passed through the fault detection module to calculate the variation of the flux linkage to determine whether the permanent magnet motor has the loss of field fault. The simulation results show that the loss of field fault detection module can achieve efficient fault diagnosis within 0.1 s by analyzing the permanent magnet flux linkage dynamically corrected by extended Kalman filter.