Due to the influence of fracture undulation, there is an obvious difference in the grout flow rule between the rock mass with rough fractures and the rock mass with existing results. Based on the parallel plate model of fluid flow in a single crack, a flow rule model of viscous fluid in a rough fracture was developed using a corrected cubic law. The derived results were agreed well with that from a single crack grouting test. Based on a discrete fracture network (DFN) model, the grouting process in a fractured rock mass was simulated and studied by changing the fracture network characteristics and the grouting pressure. The results show that the grouting diffusion volume is controlled by the permeability of rock mass, but the connectivity plays a key factor affecting the fluid diffusion process, which can affect the fluid diffusion in local areas. The increase of grouting pressure changes the slurry diffusion velocity, which can make the slurry easier to diffuse to the remote areas that far from the grouting point and those areas with poor local connectivity, however, excessive grouting pressure may cause the fracture opening and even lead to the rock mass failure.