This paper focuses on a 48-slot, 8-pole surface-mounted permanent magnet synchronous motor (SPMSM) and investigates the impact of demagnetization conditions on the electromagnetic performance and vibration noise of the motor. Firstly, the motor’s air-gap magnetic field and electromagnetic force waves are analytically calculated. Ansys/Maxwell is used for electromagnetic field simulation analysis, and the demagnetization condition is preset to obtain the air-gap magnetic flux density and radial electromagnetic force before and after demagnetization. The corresponding temporal and spatial harmonic orders are obtained through Fourier decomposition. Modal analysis is then conducted to determine the modal shapes and natural frequencies of the motor. Using Workbench, a multi-physics simulation of the motor’s magneto-mechanical-acoustic fields is performed, and harmonic response analysis is carried out to generate vibration contour maps and displacement data. The electromagnetic vibration noise under demagnetization conditions is compared with that under normal operating conditions. The results indicate that demagnetization leads to a reduction in radial electromagnetic force and output torque, and a decrease in vibration displacement and noise sound pressure level. This paper provides a theoretical reference for the analysis of motor demagnetization conditions.