Overall stability calculation for river bank slopes could not reflect the impact of water level fluctuation on various parts of the slope stability quantitatively. On the basis of the theory of unsaturated soil mechanics and local factor of safety method (LFS), we conducted numerical analysis on the stability of river bank slope by the model of unsaturated liquid-solid coupling. Taking a large-scale model test for example, we plotted the contours of LFS at different instances under water level fluctuation to reveal the causes of destruction and the processes of internal stability. Compared with actual observations, the analysis result demonstrates that 1) the LFS method could describe the river bank slope stability and its dynamic changes at various slope positions reasonably; 2) the free face of slope is of low stability, and water level rise would reduce the effective stress and lead to the loss of matrix suction in the unsaturated zone of slope, which is easy to induce shallow slide; 3) the hydrodynamic pressure formed by rapid drawdown is the main cause of the decline of slope stability and the most vulnerable zone is the toe of slope where retrogressive failures often emerges; 4) along with the increase of hydraulic gradient, the internal local stability coefficient decreases and the instability zone, which distributes mainly between the saturation line and the toe of slope, expands.