The lateral interaction between flowing fluid and flexible structure in an RT stirred tank with a diameter of 0.58 m was numerically simulated by means of a method coupling Computational Fluid Dynamics (CFD) with Computational Structure Dynamics (CSD). The results show that the characteristics of the shaft bending moment predicted by the coupling simulation, including the dimensionless mean and relative standard deviation, are in good agreement with the experimental data. Analyses of the simulation results show that the trajectory shape of impeller lateral movement depends on the impeller mass imbalance. The dimensionless lateral fluid and structural loads separated from the resulting lateral loads are both associated with the impeller speed and imbalance. Further analysis of the impeller lateral force power spectral density shows that the fluctuations of these forces are characterized by the rather low frequency, speed frequency and blade passing frequency.