Abstract: 10%, 20%, 30%, 40%, 50% and 60% uniform demagnetization faults on all poles of the magnet are considered, and the effects of different fault severity on the performance of the machine are studied. The signals of back-EMF, cogging torque and leakage magnetic field, which are directly related to the demagnetization of permanent magnet, are discussed by finite element analysis. When the permanent magnet in PMSG deteriorates, the flux will decrease accordingly, which will directly lead to the decrease of leakage field. Since the back EMF depends on the rate of flux change, the change of its back EMF can reflect the change of flux. When the direction of the magnetic field strength changes, the cogging torque generated by the trend of keeping the rotor at the minimum energy level will also change directly. Therefore, in the development of fault diagnosis technology, it is necessary to study the characteristics of these signals. The results can be a guideline for the health state detection and fault diagnosis of aerogenerator.