Cement grout, commonly applied in engineering, is a type of non-Newtonian fluids, which exhibits complex macroscopic nonlinear flow characteristics when diffusing in fractures and presents special structures such as plug flow due to the existence of yield stress. This study involved preparing artificial grouts following the Herschel-Bulkley (H-B) model and conducting visualization grouting tests on flat fractures using particle image velocity (PIV) measurements. Grout flow numerical simulations were performed using the finite element method (FEM) to solve the H-B-P (H-B-Papanastasiou) equations. The nonlinear correlation between pressure gradient and flow rate was theoretically, experimentally, and numerically analyzed and confirmed using the analytical solution of the single-phase yield-power-law for fluid flow in flat fractures. Plug flow features of H-B fluids were examined by comparing velocity profiles obtained through various methods. Comparison with the Bingham model proved that the H-B model aligns better with real grout flow. This study comprehensively verified the theory and numerical model based on an originally developed visualization method and laying the groundwork for parameter determination, which may help improve the grouting technique in complex engineering rock masses.