Abstract: There is an irreversible demagnetization risk for high-density permanent magnet synchronous motor rotor permanent magnets for industrial robots. A permanent magnet synchronous motor permanent magnet with an 8-pole 36-slot industrial robot is taken as an example to study the demagnetization fault. The time-step finite element method is used to analyze the change of electromagnetic field in different degrees of demagnetization of permanent magnet motor. The Fourier series harmonic decomposition theory is used to obtain the air gap magnetic field and current harmonic content of the motor. The air gap magnetic field, armature current, back EMF waveform, flux linkage waveform, magnetic field trajectory and the magnetic core density characteristics of the stator core are given under the state. Based on the analysis of the above parameters, the feature quantities closely related to the demagnetization fault are extracted, which lays a foundation for further research on the demagnetization fault diagnosis and online monitoring theory of permanent magnet synchronous motor.