To reveal the influence mechanism of polyurethane grouting on the crack propagation behavior of SMA-13 asphalt mixtures, this paper first conducted a semi-circular bending (SCB) test to validate the effectiveness of the PFC^2D discrete element model, which was employed to simulate the microscopic responses of mixtures under different notch positions and grouting conditions. The analysis focused on load-displacement curves, crack propagation paths, crack number evolution, and force chain distribution characteristics. The results indicate that polyurethane grouting significantly increases the peak load and peak displacement of the specimens, thereby enhancing the fracture load-bearing capacity and deformation tolerance, with the most pronounced effect occurring when the notch is located along the specimen’s symmetry axis. Crack path analysis reveals that grouting redirects the main crack away from the original notch tip toward weaker regions, thus effectively mitigating local stress concentration and improving structural integrity. Crack evolution is characterized by the three stages of “slow accumulation, rapid propagation, and gradual stabilization”, dominated by Mode I cracks. In low-offset notch conditions, grouting delays crack initiation and reduces the propagation rates. Furthermore, microscopic force chain analysis demonstrates that grouting improves the distribution of tensile and compressive chains, leading to more continuous stress transmission paths.