There are many uncertainties affecting the stability of the foundation pit, such as the size of the foundation pit, the soil properties of the site, construction quality, and environmental factors. In order to explore a scientific and rational method for evaluating the stability of foundation pit slopes, based on the unascertained measure theory, this paper selects the depth of foundation pits, supporting methods, main soil layer cohesion, main soil internal friction angle, groundwater and surface, water effect, adjacent load, adjacent load pit margin, and construction quality as evaluation indicators. The weight of each index is determined by using the three-scale method combined with the analytic hierarchy process.The stability evaluation model of the foundation pit is established. This model is applied to two examples of foundation pit engineering. Compared with actual monitoring results, the reliability of evaluation results is credible. The results show that the foundation slope stability evaluation model based on unascertained measurement theory is feasible.