The composite mine slopes formed by transitioning from underground to open‐pit mining are affected by historical shaft disturbances and complex geological structures，resulting in significant heterogeneity in the rock mass structure. Traditional stability analysis methods struggle to accurately characterize their failure probability. To address the challenge of solving complex reliability functions，an innovative method was proposed for calculating the reliability index β based on the Logistic iterative equation.This method dynamically optimized the iterative step size and convergence threshold，significantly enhancing the computational efficiency and stability of the gradient descent algorithm and effectively solving the reliability calculation problem for high‐dimensional nonlinear limit state equations. By taking the typical underground to open‐pit mine，the Shizhuyuan Mine，as the research object，the geological zoning of the mine slope was performed based on its engineering geological conditions. Bishop and Janbu limit state hyperplane models for the final pit slope were constructed，enabling precise calculation and study of the slope reliability. The results show that the reliability values of all slopes in the Shizhuyuan Mine exceed 2.42，meeting the national standard for highway slopes，thereby confirming the stability of the final pit slope.