Abstract: Uniform demagnetization is a common demagnetization fault type of permanent magnet synchronous motor (PMSM).Through the analysis of the performance of the motor under different demagnetization degree of permanent magnet, the back-EMF radius of gyration method was proposed to diagnose the uniform demagnetization fault of the direct-drive permanent magnet synchronous motor (DDPMSM).Time delay embedding (TDE) technique to obtain the coordinate data of the differential phase space figure of the back-EMF, and the back-EMF radius of gyration model of the back EMF was established.Through the simulation and calculation on the back-EMF radius of gyration with a different operating speeds and different faults DDPMSM, the linear relationship between the degree of demagnetization failure and radius of gyration was obtained.The least squares method was used to fit the mathematical equations to the three parameters of demagnetization degree and radius of gyration and speed.The feasibility and effectiveness of the DDPMSM uniform demagnetization fault diagnosis method were proved by simulation results.