In order to study the bearing friction characteristics of the RV reducer, an experimental study was put forward to investigate the influence of different preloads on the gear friction. The preload was achieved by adjusting the clearance of the sun gear shaft bearing. Then, the test was conducted under three different operating speeds. Firstly, the transmitted power and torque were analyzed to study the power loss due to abnormal friction. Then, a statistical analysis of the vibration signal was carried out to study the change in RV vibration. Moreover, the spectrum of the RV vibration was obtained to study the dynamic characteristics of the RV reducer. The results show that with the increase of the bearing preload, the meshing state of the sun gear and the planet was deteriorated, which cause the obvious increase in the resonant band in the vibration spectrum. The RMS value of the amplitudes of the resonant band can be an indicator of the diagnosis of abnormal friction in the RV reducer.