Abstract: According to the design and geological condition of Da-ping tunnel, numerical methods used to analyze the mechanical responses of tunnel with large cross section on surrounding rock condition of class Ⅲ. Then stress field 、meet an emergency field 、displacement field and mechanical responses of shotcrete and bolt support of different construction methods are studied, and base on compare synthetically step three branch excavating method is the rational construction of Da-ping tunnel surrounding rock condition of class Ⅲ.