<p>A non-proportional coordinated strength reduction method is proposed to accurately capture the degradation of mechanical and deformation parameters in water-weakened rock slopes, effectively reflecting the observed weakening behavior of rocks under the influence of water. The proposed method is applied to strip-mining operations at the Huolinhe open-pit coal mine in China to investigate slope displacement and stability, establishing the relationships between excavation width, slope displacement, and stability. Laboratory rock experiments provide the parameters for the proposed strength reduction method, while site excavation data inform the numerical simulations. The failure process of strip-mining slopes under water-weakening conditions is categorized into four distinct stages. Numerical simulations demonstrate a nonlinear decrease in the safety factor for upper arching slope failure as excavation width increases. The arching failure height in the upper slope area follows a nonlinear trend, initially decreasing and then increasing as the excavation width increases. Furthermore, a displacement curve illustrating slope instability due to water weakening is obtained through numerical simulation. The research findings provide important insights into the displacement patterns and failure mechanisms of slopes influenced by water, providing valuable guidance for engineering practice and the formulation of effective risk mitigation strategies.</p>