PU/WG (polyurethane/water glass) grouting material has good injectability, fast curing speed, and low grouting pressure. Combined grouting of foam and reinforcement PU/WG grout is suitable for water blocking and reinforcement in deep water-rich fractured rock masses. In order to study the diffusion law of PU/WG grout in combined grouting, the Combined grouting of PU/WG grout test was carried out by the self-developed visual penetration grouting simulation test system. The dynamic evolution of the pressure field and diffusion pattern in PU/WG combined grouting were analyzed and verified based on the porous media multiphase flow module and Darcy’s law module in COMSOL. The results of model tests and numerical simulations were compared. The pressure changes at different positions and diffusion mechanisms in combined grouting were analyzed. The results show that the required grouting pressure level for foam PU/WG grout is relatively low in combined grouting, equal to the head pressure in the environment. Foam PU/WG grout can effectively fill the pores and make them no longer connected, blocking flowing water diffusion paths. Foam PU/WG grout forms a water-blocking barrier with medium, turning flow water into static water and blocking pressure transmission. Under composite grouting, the reinforcement PU/WG grout can effectively penetrate the pores of the medium and the foamed grout, improving the grouting quality. The diffusion range of combined grouting is controllable. Some suggestions were put forward for the design and application of combined grouting.