To investigate the deformation control and support optimization methods of surrounding rock in a large longitudinal slope tunnel with a fault zone, by considering the maximum design inclination angle of the arc section of the flood spillway tunnel, geological parameters of the fault, and support parameters such as anchor rods and steel mesh, and with reference to the Saint-Venant’s principle to control the model boundary, the finite element method was used to establish an analysis and calculation model for the construction support of the tunnel. The deformation of the surrounding rock and the stress on the support structure were calculated and dynamically analyzed. The results show that the stability of the surrounding rock of the large longitudinal slope tunnel in the fault zone is poor. The vertical displacement of the surrounding rock at the fault increases by 0.8 to 1.5 times compared with the complete stratum, and the horizontal displacement increases by 1.0 to 1.2 times. It is necessary to strengthen the initial support strength of the large longitudinal slope tunnel in combination with the engineering practice. The control effect of surrounding rock pressure of steel arch erected in vertical direction is better than that in perpendicular direction. Increasing the density of steel arch can effectively reduce its stress. The maximum displacement values of the vault, arch bottom and side wall under the optimal steel arch erection method are reduced by 17.8%, 36.31% and 18.54% respectively compared with the results of numerical simulation. The on-site monitoring results show that the steel arch support has a significant effect.