In view of the problems existing in the evaluation process of slope stability in cold regions, the improved entropy weight method and extension theory are used to evaluate the slope stability. Nine indexes which affect the slope stability in cold regions are selected, the matter element system of slope stability evaluation is established by extension theory, and the extension correlation degree between evaluation index and evaluation grade is calculated. The improved entropy weight method is used to calculate the entropy weight of the matter element index system, and then the stability grade of the slope matter element is determined. Based on the extension theory and improved entropy weight method, the slope stability of Beizhan open pit mine is predicted. The results show that the prediction results are consistent with the actual situation of the project, and the evaluation method can be applied to the slope stability evaluation in cold regions. By using extension theory and improved entropy weight method, the evaluation index can be changed from a single determined value to an interval value, and the influence degree of the evaluation index can be analyzed more comprehensively. This method improves the evaluation accuracy and provides a new method for the slope stability evaluation in cold regions.