In order to clarify the optimal slope angle for the sectional excavation of tunnel faces in the new micro-disturbance mechanical tunneling method for weak surrounding rock, this study a tunnel along a highway in Sichuan is studied. By integrating soil theory with the Mohr-Coulomb criterion to establish a limit equilibrium model and combining with numerical simulations to conduct sensitivity analysis of Class IV and Class V surrounding rock parameters and limit equilibrium verification methods, the optimal design of the tunnel face slope angle is systematically studied. The results indicate that for Class IV surrounding rock, when the safety factor meets the code requirements, the slope angle of the tunnel face exhibits significant stratigraphic dependency, with a variation range of 55.0° ~60.5°. For Class V surrounding rock, it is required to ensure a limit equilibrium safety factor of the tunnel face ≥1, the excavation slope angle must be controlled within 35.2°, and mechanical support measures must be applied simultaneously as the slope angle increases. The findings of this study can provide a theoretical basis for the slope angle design and stability control of mechanized construction in tunnels with weak surrounding rock.