Due to its complexity, collapse during the construction of tunnels is a hot issue. A dynamic risk assessment methodology based on a new attribute interval assessment model was presented, which consists of pre-assessment before excavation and post-assessment after excavation and before primary support. The surrounding rock level I₁, rock mass integrity I₂, buried depth I₃, bias angle I₄, groundwater I₅, and construction factors I₆ were selected as assessment indices. The value of every evaluation index is an interval rather than a definite value. On the basis of the traditional attribute assessment model, the integration method was introduced to compute the single index attribute measure of the interval. The index weight was determined by employing the combination weighting method, including subjective weight based on frequency statistic method, and objective weight based on analytic hierarchy process (AHP). The proposed method was applied in the right Duanjiawu Tunnel. The results were good agreement with actual excavation situation and the results of other methods, which proved the science and feasibility of this method.