When constructing tunnels in soft rock area with high ground stress, due to the soft rock has low strength and strong expansibility, and is extruded by high ground stress. Therefore, it is easy to cause large deformation of soft rock if the construction measures are not appropriate, which brings great difficulties to the engineering construction. Based on the theory of surrounding rock loose circle, the unified strength criterion is adopted to analyze the stress state of surrounding rock with considering the influence of intermediate principal stress. Then the radius calculation formula of loose circle of surrounding rock of large deformation soft-rock tunnel is obtained. Based on the Elastic-plastic analysis of surrounding rock of Xiejiapo tunnel of Anlan Expressway, it is found that the distribution of surrounding rock loose circle of large deformation soft-rock tunnel is not uniform along the cross section. And the loose circle tends to be large in the side wall and small in the vault, and increasing with the increase of large deformation grade and the decrease of support reaction force. Combined with field test, it is found that the thickness of loose circle of grade II large deformation is 6.5~7.0m at vault and 7.0~7.5m at side wall; the thickness of loose circle of grade III large deformation is 7.5~8.0m at vault and 8.0~8.5m at side wall; and optimize the length of the system bolt based on the thickness of the loose circle. By monitoring the optimized section, it can be seen that the deformation of surrounding rock is reduced significantly and the stability is improved effectively.