The distortion of air gap magnetic field caused by the rotor eccentricity contributes to the electromechanical coupling vibration of the brushless DC (BLDC) permanent magnet in-wheel motor (PMIWM) in electric vehicles (EV). The comfort of the BLDC in-wheel motor drive (IWMD) EV is seriously affected. To deeply investigate the electromechanical coupling vibration of the PMIWM under air gap eccentricity, the PMIWM, tyre and road excitation are analyzed first. The influence of air gap eccentricity on air gap magnetic density is investigated. The coupling law of the air gap and the unbalanced magnetic force (UMF) is studied. The coupling characteristics of eccentricity rate, air gap magnetic density, UMF, phase current and vibration acceleration are verified on the test bench in the laboratory. The mechanism of the electromechanical coupling vibration of the BLDC PMIWM under air gap static eccentricity (SE), dynamic eccentricity (DE) and hybrid eccentricity (HE) is revealed. DE and HE deteriorate the vibration acceleration amplitude, which contributes the electromechanical coupling vibration of the PMIWM. The research results provide a solid foundation for the vibration and noise suppression of the PMIWM in distributed drive EV.