The grout propagation plays a crucial role in the design of polyurethane grouting for reinforcing and sealing fractured rock. Understanding the influence of isocyanate index on the diffusion process is essential for selection and material modification to improve the grouting effectiveness. In this study, a numerical method was developed to simulate the grout propagation of polyurethane, considering the coupled of reaction kinetics and fluid flow. Furthermore, the influence of the isocyanate index on pressure, diffusion distance, component conversion and density were analyzed. These findings contribute to the preparation and selection of polyurethane grout for practical engineering applications.