Abstract: The study comprehensively considered 8 risk factors affecting the stability of gangue slope by applying extension theory, and established a new risk assessment model for the stability of gangue slope. Compared with the traditional extension theory-based evaluation model, the new model is improved in the calculation of extension distance, and it uses subjective and objective analysis method to comprehensively analyze 8 risk weight indices, thus it can reduce the systematic error of the traditional theory algorithm in the risk evaluation of slope stability. Taking Wangjialing gangue slope as a case, the risk assessment for slope stability concludes that the characteristic value of risk level increases from the original 1.284 at July 7th, 2020 to 2.263 at August 14th, 2020, and the risk level increases from low risk to medium low risk, thus achieves a quantitative risk assessment of coal gangue slope. Through applications in a number of gangue slope cases, it is concluded that the instability risk analysis based on improved extension theory evaluation model can be used in combination with existing slope stability assessment tools, and realize the comprehensive evaluation of mine slope safety index and risk index. In addition, by analyzing the on-site monitoring data index, the method can provide an objective and effective risk analysis method for mine slope, and improve the current situation that slope monitoring pays more attention to data acquisition than data analysis.