Abstract: Problems including poor stability of surrounding rock, rock softening in water and large deformation of soft rock are encountered during the construction of Fenghuangshan tunnel in No.6 bid section of Chuxiong section of Water Diversion Project in Central Yunnan. In this paper, firstly, the physical and mechanical characteristics of soft rock in the large deformation area are obtained by carrying out the indoor mechanical test on typical soft rock samples in the area, and the deformation characteristics are known by analyzing the failure of the tested soft rock;secondly, FLAC 3D numerical simulation software is used to calculate the stress and strain changes of the tunnel in the large deformation section, and the characteristics of the large deformation of the Fenghuangshan tunnel are mastered; finally, the FLAC 3D simulation results are compared and verified with the field monitoring data, and the comparison results are within the allowable range of error, proving the accuracy of the numerical simulation.