Abstract: The tidal level fluctuations in the estuarine area trigger fluctuations in the water level of the coastal diving layer, exhibiting a nonsynchronous fluctuation pattern with the tidal level. This phenomenon has a profound impact on the stability of the bank slope. The varying water levels disrupt the equilibrium and increase the risk of slope instability, posing significant challenges to the structural integrity and long-term stability of the slope. During the ebb tide stage, the groundwater table within the interior of the bank slope tends to be higher than the tidal level. Consequently, this leads to a greater water holding capacity in the saturated area and creates conditions that are more susceptible to inducing bank slope collapse and instability. The higher water levels during this stage increase the potential for instability, posing a significant risk to the structural integrity and stability of the bank slope. In order to address the specific challenges of complex estuaries, this paper presents SUBS, a bank collapse warning model. SUBS is developed by improving the BSTEM（Bank Stability and Toe Erosion Model） and coupling it with SUTRA（Saturated-Unsaturated Transport）, a groundwater model that incorporates unsaturated flow movement. By combining these advancements, SUBS provides a comprehensive and accurate framework for predicting bank collapse events in intricate estuarine environments, contributing to improved bank collapse risk assessment. The research findings show that the fluctuation of groundwater caused by the fluctuation of tidal level in estuarine area has quite a strong hysteretic property. The Factor of Safety of the bank slope and the most dangerous failure surface will change periodically with the tide. And the Factor of Safety will increase during the rising tide, reflecting improved stability. Conversely, during the ebb tide, the Factor of Safety decreases due to the lag in groundwater drainage. Especially during the ebb stage of the spring tide, the water level plummets rapidly, significantly heightening the risk of bank slope collapse. Additionally, the presence of seepage faces on the bank slope further increases the vulnerability to instability. By comparing the stability of homogeneous and composite bank slopes, the results strongly indicate that soil cohesion plays a significant role in determining the stability of the slope. This finding underscores the importance of considering the cohesive properties of the soil when bank slope stability is assessed. The stability of the bank slope is significantly enhanced in the composite structure, where clay is present on the upper part and sand on the lower part. This configuration outperforms the bank slope composed of homogeneous sand with low cohesion. The presence of clay provides increased cohesion, resulting in improved resistance to slope failure and greater overall stability of the composite bank slope. In this context, more attention should be paid to the influence of groundwater level fluctuation. This paper expands the application scope of the traditional BSTEM model by coupling a groundwater model and considering real-time fluctuations in groundwater levels. It introduces a new research method for predicting the slope stability in estuarine areas, thereby enhancing our understanding and analysis of slope behavior in these complex environments.