[Objective] The vulnerability assessment of subgrade was studied in order to provide an effective method for the determination of prevention rank and the assessment of the disaster-bearing capability of subgrade.[Methods] According to the subgrade suffering from debris flow and previous research, the concept of subgrade vulnerability was proposed. Based on the characteristics of the subgrade and debris flow, the influences of the subgrade geological location, the design, the construction and the operation maintenance on subgrade vulnerability were systemically analyzed. Then the evaluating indicators of subgrade vulnerability were determined. The weights of indicators at all levels were determined by using the AHP(analytic hierarchy process) method, and the ridge distribution function was used to calculate the membership grades of indicators. The assessment model of subgrade vulnerability was finally established based on the fuzzy comprehensive evaluation.[Results] The established model was applied to assess one subgrade of the highway and the result was moderate vulnerability.[Conclusion] The evaluation result was consistent with the actual situation, and confirmed the feasibility of the method.