Abstract: The grouting in water-rich fractured rock masses is a process in which the pressurized grouts gradually displace the existing water. It is important to thoroughly investigate the grout-water displacement laws for improving the engineering grouting efficiency. In this study, a visualization technique that incorporates the particle image velocimetry (PIV) into the grout-water displacement tests is established, and is used to capture the flow field distribution in a 3D-printed transparent rough-walled fracture along with the flow velocity and hydraulic pressure measurements. The Navier-Stokes equations are solved based on the finite element method to simulate the displacement process, and the simulation is compared with the experimental observations. The results show that under the constant flow rate, the injection pressure first increases gently, followed by a rapid increase stage, and finally approaches a constant value. The grouts preferentially flow through some major channels, and the injection pressure tends to increase gently after the grout reaches the outlet. The residual water is mainly distributed in the dead end close to the edge of main flow channels and the locations where sudden changes in aperture happen. The parallel-plate model can underestimate the injection pressure by up to 45% comparing to the corresponding rough-walled model. It is therefore necessary to consider fracture roughness in the theoretical assessment of grouting pressures to achieve better grouting performance.