Abstract: Based on geologic features of debris flow fan, combined with the engineering background of Futang tunnel in Dujiangyan to Wenchuan expressway, a 3D numerical model had been established to study tunnel excavation and support in debris flow fan. The paper presents different modelling conditions, including the non-support status, designed support status, optimizing support status as well as the deposition and erosion of debris flow fan. The study results show that, tunnel excavated in debris flow fan, the vault settlements are main controlling factor and lead to subsidence and side wall convergence; after pointed optimizing treatment to vault settlement using the advanced grouting and the systematical anchor lengthening, deformation of tunnel lining and subsidence are reduce markedly, it shows that the optimizations are effectual and reasonable. In addition, if the deposition thickness of debris flow above tunnel vault is less than 14 m, the tunnel is in obvious unsafe condition; if the debris flow thickness is more than 42 m, the stability of surrounding rock will become better and be easy to form a stabilizing system.