In order to study the applicability of a large crosssection tunneling method, microbench method Ⅰ, for rapid excavation construction in Grade Ⅳ surrounding rock to, the stability of surrounding rock during the construction of microstep I method is analyzed based on numerical analysis of strength reduction method and displacement mutation and calculation nonconvergence as main indices; and the safety of the actual tunnel structure is determined by analyzing the field monitoring data. The analytical results show that: (1) The vertical displacement of the crown top and spandrel is sensitive to the strength reduction factor, while the horizontal convergence of arch feet and sidewall is sensitive to the strength reduction factor. (2) The strength reduction safety factor obtained by using the traditional threebench method and the that by microbench method Ⅰ with large machinery is almost the same; and the microbench Ⅰ method has little influence on the stability of the surrounding rock, which can ensure the safe and rapid tunneling and has good feasibility in Grade Ⅳ surrounding rock.