During excavation using the cut blasting method in deep rock masses, there are difficulties resulting from the in-situ stress influences. This study uses numerical simulation methods to assess the causes of the difficulties encountered in cut blasting. In order to overcome this difficulty, the Riedel–Hiermaier–Thoma (RHT) model in the LS-DYNA software was employed. In the simulation, the parameter determination for the RHT model was first carried out based on existing experimental data. Additionally, the existing blasting experiment was used to verify the determined parameters of RHT model. Second, the RHT model was adopted to investigate the damage mechanisms of cut blasting under different hydrostatic pressures and different lateral pressure coefficients. The simulation results indicate that the main causes of the complications arising in deep rock mass excavation are resistance to in-situ stresses and anisotropy in the damage propagation direction. Third, in order to overcome such difficulties, a cut blasting design optimization was conducted for a 2525 m depth of rock mass. According to the numerical simulation of this optimization, a modified cut blasting design method applicable to deep rock mass was proposed. This study can provide solutions to the cut blasting difficulties that are encountered during the excavation of deep rock masses.