In view of the demagnetization fault of the permanent magnets of the permanent magnet synchronous motor (PMSM) rotor, a flux reconstruction method has thus been proposed based on the integral sliding mode observer. Firstly, the current state equation is established of the permanent magnet synchronous motor during demagnetization fault in the d-q axis coordinate system. Secondly, an integral sliding mode observer is constructed with the characteristics of the sliding mode observer and the proportional integral observer combined together, thus realizing the on-line detection and reconstruction of flux linkage information of the permanent magnet synchronous motor (PMSM). Finally, the stability of the integral sliding mode observer is verified based on Lyapunov stability theory, with the permanent magnetic flux linkage reconstructed according to the sliding mode equivalent control principle. The simulation results show that the designed integral sliding mode observer is characterized with a high observation accuracy for flux linkage, greatly improved in its robustness and rapidity.