Tunnel excavation is actually an unloading process. Due to the influence of unloading stress paths, the parameters of the surrounding rock in the plastic zone of the tunnel are degraded and the mechanical parameters are reduced relative to the unexcavated state. In this paper, a model for calculating the reduced parameters of the surrounding rock in the plastic zone of the tunnel is obtained based on the typical soft rock-mudstone unloading triaxial test of the tunnel by other scholars. Comparing the results with the traditional Fenner formula, it is found that this model is more consistent with the results of excavation and support calculations for soft rock tunnels under high ground stress. The model is solved theoretically using the Euler explicit differential solution algorithm to study the effects of different support forces, cohesive forces, friction angles and deformation moduli on the radius of the tunnel plastic circle and the displacement of the perimeter of the tunnel. The model was developed using FISH language and embedded in FLAC 3D software to calculate the effect of different support timing on the support effect. Finally, the tunnel calculation model is validated based on the field measurement data of Longxi tunnel. The research results can provide a theoretical basis for the design of high ground stress soft rock tunnels.