The process of stress-induced fracturing around underground excavations is associated with dilatancy-bulking behavior of the fractured rock. Due to the importance in earth science and engineering design, this paper presents an experimental investigation of rock dilatancy-bulking behavior based on triaxial loading and unloading tests carried out on weak sandstone samples from the Gubei colliery. Through experimental and theoretical analyses, it is found that the volume increase of stress-fractured rock can be divided into two groups: (1) pre-peak dilatancy due to crack opening and propagation, (2) post-peak bulking by the sliding, rotate or detachment of rock blocks. And the post-peak bulking deformation is the main cause of the rock volume expansion. Moreover, the evolution of characteristic stresses and dilation angle show strong pressure dependence, which means that confining pressure strongly inhibits dilatancy-bulking behavior. It also indicates that the rock response under loading and unloading conditions is substantially different.