In this paper, the stability of the tunnels in an underground mine was investigated by three-dimensional numerical modeling. Based on the available geological, geotechnical, and mine construction information, a three-dimensional model was built using the 3DEC code. The model contains the features of the inclined lithologies, a large-scale non-persistent fault, and a complex tunnel system. Stress analyses were performed on the rock mass having the elastic-perfectly plastic and strain-softening behaviors, respectively. The effects of the horizontal in situ stress and the support system were studied through the distributions of stress, displacement, and yielded zone around the tunnels. By comparing the numerical modeling results with the field deformation measurements, it was found that the strain-softening cases with the lateral stress ratios of 0.5 and 1.0 are accurate and applicable for this mine.