Aiming to explore the variation law of surrounding rock displacement and internal force of supporting structure during the excavation of carbonaceous slate tunnel under high stress, based on the Muzhailing Highway Tunnel of Weiwu Expressway, the finite difference software FLAC3D was used to establish the numerical model of carbonaceous slate tunnel under three-bench seven-step excavation method, and three supporting schemes of strong, medium and weak were proposed. The influence of excavation distance on the displacement of surrounding rock and the internal force of supporting structure was analyzed, and the rationality of supporting design parameters was evaluated. The results show that the cumulative settlement and convergence deformation of surrounding rock during excavation are compared: strong support < medium support < weak support. Under the same distance of excavation, with the increase of support strength, the elastic resistance provided by support is stronger, and the settlement and convergence deformation of surrounding rock decrease. Taking the strong support as an example, the axial force of the supporting structure shows a trend of increasing first and then decreasing. Finally, the axial force continues to rise but the trend slows down and the excavation distance is 50 m, it tends to be stable. In the evaluation of supporting design parameters, when the excavation distance (50 m) is over, the minimum safety factors of steel frame compression in strong, medium and weak schemes are 3.903, 3.718 and 3.264 respectively. The steel frame of strong support at # 1 vault is destroyed earlier than that of shotcrete, and the minimum safety factors of concrete are 3.491, 2.987 and 2.666 respectively. The shotcrete at # 2 left arch waist is destroyed earlier than that of steel frame, so the medium support scheme is selected, material I25b × C25, initial lining thickness 26 cm, steel frame distance 0.8 m. The research results can provide some reference for the excavation and support structure design of similar soft rock tunnels.