To avoid the influence of partial demagnetization of permanent magnet on the control accuracy and safe operation of permanent magnet synchronous motors (PMSMs), a method to monitor partial demagnetization faults for fractional slot concentrated winding (FSCW) PMSM based on back electromotive force (EMF) harmonic analysis is proposed. After theoretical derivation, the mathematical models of magneto motive force (MMF) and single-slot back EMF under demagnetization are obtained. Then, with the amplitude of each harmonic component of the single-slot back EMF and winding coefficients of FSCW-PMSM, the variation of each harmonic amplitude of the single-phase back EMF can be deduced. The theoretical derivation above can verify the feasibility of judging the demagnetization fault by the variation of single-phase back EMF′s harmonics. Finally, using the prototype parameters, taking a 20-pole 24-slot interior PMSM to build a finite element model, a 2D electromagnetic simulation model is established to obtain the variation of the characteristic harmonics of the single-phase back EMF. The results show that the characteristic harmonic of single-phase back EMF changes periodically with the increase of the number of faults.