Abstract: In order to study the unstable vibration of generator unit in nuclear power plant, a rotor dynamics analysis model was established, in which the thermal effect induced by oil shear flow in bearing was considered. One-dimensional energy balance equation was used to solve the journal temperature difference caused by the flow of lubricating oil in the bearing small gap. The average journal temperature difference and its angle during a whirling cycle were obtained. The effect of cross-sectional temperature gradient on shaft bending deformation was calculated, and the equivalent bending moment was obtained. The equivalent bending moment was input to the shaft in finite element model, and the bending deformation of the generator shaft was calculated. The results show that the temperature difference of the journal is induced by the shear flow of lubricating oil in the bearing clearance. The temperature difference of the journal increases with the increase of journal eccentricity and bearing clearance. After the eccentricity of the journal increases to a certain extent and the bearing clearance decreases to a certain extent, the temperature difference of the journal section increases rapidly, exhibiting typical nonlinear characteristics. If the amplitude of synchronous whirling orbit increases, the dynamic eccentricity increases and the cross-sectional temperature difference increases. Journal temperature difference of a certain nuclear power generation unit was calculated using the model. Within a reasonable range of bearing parameters in nuclear power units, the temperature difference can reach 10 ℃. Its impact on vibration is equivalent to the impact of G2.5 level imbalance force on the generator. The periodic vibration fluctuation phenomenon that occurred on a certain type of nuclear power unit was introduced. Test shows that the fluctuation amplitude is related to the lubricating oil temperature. It is pointed that the journal temperature difference produces rotational unbalance, resulting in vibration fluctuation. Unstable vibration is related to lubricating oil viscosity, bearing clearance, eccentricity ratio and unbalanced force. The unstable vibration can be reduced by reducing the viscosity of lubricating oil, increasing the bearing clearance, reducing the eccentricity and unbalance. The engineering verification test of adjusting lubricating oil temperature was carried out. The test results show that increasing the temperature of lubricating oil to reduce the temperature viscosity of lubricating oil is helpful to reduce the unstable vibration. The analysis results by calculation are consistent with the engineering practice. The model established in this study can be used for the analysis of unstable vibration of nuclear power generator units.