Noise, vibration, and harshness (NVH) in vehicle have received increased attention in the recent years. Excessive noise and vibration will shorten the service life of the instruments and reduce the ride comfort, make driver fatigue, affect safety. Compared with DF Series locomotive, a pivot-mount isolation system is applied to HXN3 diesel locomotive cab vibration isolation system. But the vibration of HXN3 diesel locomotive cab is always excessive. This paper analyzes the reason why HXN3 diesel locomotive cab vibration isolation system does not work via establishing the dynamic equations of cab, calculating the vibration frequency response function (FRF).