Abstract: The external rotor permanent magnet generator (ERPMG) is frequently accompanied by air gap eccentricity in operation, and it is of great significance to explore the difference in generator mechanical response under various types of air gap eccentricity faults for the purpose of monitoring the generator operation status and vibration suppression. This paper comprehensively analyses the trends of air gap magnetic flux density and unbalanced magnetic pull force (UMPF) under air gap static eccentricity, air gap dynamic eccentricity and air gap hybrid eccentricity faults. It systematically reveals the stator vibration response characteristics based on the stator core vibration model. Firstly, analytical expressions for the UMPF under normal conditions and different air gap eccentricity faults are derived. Moreover, a finite element simulation model is established in order to elucidate the stator vibration excitation force and acceleration response characteristics, as well as the structural response trend through magnetic-solid coupling. Finally, the theoretical analysis and simulation results are verified using the ERPMG fault simulation unit, and the conclusions of the three are found to be consistent. The results indicate that the stator vibration acceleration under the air gap static eccentricity is predominantly characterised by even harmonics, while the stator vibration acceleration under the air gap dynamic eccentricity and the air gap hybrid eccentricity appears to exhibit rotational frequency modulation harmonics. The findings of this study offer a valuable reference point for the analysis of air gap eccentricity fault and stator vibration response characteristics.