In order to clarify the nonlinear catastrophic failure mechanisms of unstable rock masses on steep reservoir banks under periodic water-level fluctuations in the Three Gorges Reservoir area, a numerical investigation was conducted considering the heterogeneous mechanical properties of the rock mass. Based on comprehensive field investigations, Gaussian random fields were employed to characterize the spatial variability of the physical and mechanical parameters of the Banbiyan unstable rock mass in Wushan Gorge. The strength reduction method was applied to evaluate the stability of the rock mass under different reservoir water-level conditions. The results show that considering parameter randomness leads to fluctuating stability outcomes, and the distribution of the critical failure surfaces becomes more extensive, indicating the controlling influence of structural planes on overall stability. It is found that the average factor of safety decreases by 2%~5%, with a maximum reduction of 12% at the 175 m water level. Moreover, repeated water-level fluctuations intensify the spatial variability of rock and soil properties, thereby increasing the uncertainty of collapse initiation. It is concluded that the random field-based approach provides a more realistic representation of the dynamic response of unstable rock masses on reservoir banks under complex hydro-mechanical environments, and offers valuable reference significance for risk management and stability evaluation in the Three Gorges Reservoir and similar engineering settings.