With the innovation of tunnel construction techniques and mechanical equipment, mechanical drilling and blasting for mountain tunnels have gradually developed. However, several challenges exist during the mechanical entry excavation for mountain tunnels, including the significant influence of geological factors, limited equipment adaptability, and constrained working space. This research, based on a specific tunnel project, utilized FLAC^3D finite difference software to study the mechanical excavation method and auxiliary reinforcement measures for shallow-buried segments at the large cross-section tunnel portal. A comparative analysis was conducted on the displacement of surrounding rock, the stress of supporting structures, and the variation of plastic zones before and after tunnel face reinforcement for different advances per cycle and faces when employing a micro-step construction method. The findings indicate that without face reinforcement, to ensure construction safety, the mechanical excavation advance in the tunnel portal should be controlled within 1.2 m per cycle, and the overlapping length of the forepoling pipes should be no less than 3 m. When increasing the advance per cycle, tunnel face reinforcement is necessary. With the use of 10-meter-long and 1.5-meter-spaced fiberglass anchors arranged in a honeycomb pattern to reinforce the tunnel face, the maximum advance per cycle can be increased to 1.8 m.