Large deformation often occurrs in soft rock tunnel with high ground stress during construction. Threebench excavation method and composite support structure of 3 layers of primary support + grouting through small duct + secondary lining are used to control large deformation of a soft rock tunnel with high ground stress. The mechanical characteristics of surrounding rock deformation and supporting structure are studied by site monitoring，numerical simulation and theoretical calculation. The results show that: 1) The deformation speed of surrounding rock is large when excavating top heading and intermediate bench; the deformation of surrounding rock becomes stable after construction of invert arch closure and the third primary support. 2) The stress of surrounding rock can be improved effectively by 3 layers of primary support; and the primary support structures are under compressive stress; the maximum vertical stress occurs in haunches and sidewall and the maximum horizontal stress occurs in crown top. 3) The safety factor of secondary lining can meet the requirements of related specifications and guarantee the safety of tunnel. 