[Objective] To provide a basis for studying the vulnerability of subgrade under the action of debris flow, model test is used to research the scouring and silting mechanism of the debris flow on the subgrade.[Methods] The debris flow gully in China-Pakistan K1561 highway was selected as the research prototype. A similar test model of the debris flow gully was built. Test data was obtained from the simulation of the prototype valley. The scouring and silting mechanism of highway subgrade was studied by control variate method, based on the model tests performed on 6 factors of 25 groups.[Results] The results of prototype gully model test were in good agreement with the field investigation results. Five factors, namely circulation area slope, the distance between the subgrade and the mountain pass, the maximum amount, the severity and the frequency of debris flow, showed obvious impacts on the subgrade vulnerability, while the influence of the angle between the main channel and the subgrade was not significant.[Conclusion] Five factors have important influences on subgrade vulnerability, and can be used as important evaluation indexes for debris flow vulnerability assessment.