Abstract: The actual deformation of a primary lining is far greater than the elastic deformation of a tunnel, and the plastic deformation occurs later. In this paper, surrounding rock is quantified by taking the primary lining as an elastic-plastic material and adopting the FEM numerical limit analysis method, and the interaction between the rock and lining is studied with a characteristic line method. The results show that the safety factor of the surrounding rock remains unchanged under a small stress release rate, while support efficiency decreases under a stress release rate exceeding a certain value and therefore the safety factor of the surrounding rock is reduced. A method for calculating the safety factor of surrounding rock after a stress release is proposed based on the FEM strength reduction and characteristic line methods, providing a theoretical basis for tunnel design.