A new Underground Research Facility (URF) is planned to be constructed in the Rožná Mine, Czech Republic. It is essential to evaluate the feasibility of the current excavation design before the start of the excavation. Numerical simulation using the software FLAC3D was adopted to investigate the stability or the strength-stress ratio of the URF. Laboratory tensile strength and uniaxial compressive strength tests on the rock specimens at various inclinations of the weak planes combined with the Geological Strength Index (GSI) system provide reliable deformation and strength input parameters for the explicit continuum model. The well-known modified Hoek-Brown constitutive model was employed for the numerical evaluation of the URF stability. In situ three-dimensional stress tensors were measured from the field and applied to the model boundaries. Displacement/convergence and distribution patterns of the stress state and strength-stress ratio were analysed. It was found that the current URF layout is on the safe side and the total displacement is minor and acceptable. Locations with potentially low strength-stress ratios were mainly around the roadway-chamber intersections. The current study can provide a valuable reference to similar numerical works since such large-scale numerical simulations are rare, due to the complexity and the time-consuming nature of the study.