In order to establish the essence of the strength reduction theory, the authors compare various DruckerPrager yield criteria and select a formula that best serves their needs. Taking the construction of Shengrenshan tunnel as an example, the authors adopt site monitoring together with 3D numerical simulations to analyze the stability rules of rock mass in different zones. The study shows that: 1) The unloading effect of the excavation at the invert is more evident than that at the crown tunnel; The surrounding rock at the crown goes through a compressingshearing, tensioningshearing, and compressingshearing circulating failure process; The failure of the surrounding rock at the sidewall takes a “compressingshearing” pattern, with a loosening zone occurring in the shape of an “ear”; The surrounding rock at the invert continues to be in a tridirectional tensioning state as the tunnel face passes the target section. 2) The selection of different DruckerPrager yield criteria in the numerical simulation has little effect on the calculation results of Lode angle, but it has great effect on the calculation results of stress. It is necessary to take the stress state of the surrounding rock into consideration so that a suitable DruckerPrager yield criterion can be adopted.