When constructing tunnels in soft rocks with high ground stress, it should be considered that the soft rock has low strength and strong expansibility, and is extruded by high ground stress. Therefore, it is unavoidable 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 consideration of 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 case 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 increases with the increase of deformation grade and decrease of support reaction force. Combined with field test, it is found that the thickness of loose circle of grade Ⅱ large deformation is 6.5-7.0 m at vault and 7.0-7.5 m at side wall; the thickness of loose circle of grade Ⅲ large deformation is 7.5-8.0 m at vault and 8.0-8.5 m at side wall. Therefore, the length of the system bolt should be optimized 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.