Polyurethane grouting is a commonly employed technique for seepage prevention and reinforcement during tunnel construction. Gaining insights into the propagation behavior of polyurethane grout within rock fractures holds importance in guiding grouting operations. To investigate the effects of fracture aperture on the propagation of self-expanding polyurethane grout, experimental studies were first conducted to examine the polyurethane propagation characteristics in homogenous fractures. Subsequently, propagation mechanisms of polyurethane grout and cement grout were compared and analyzed by numerical simulation. Finally, the scanning electron microscope (SEM) tests were performed to analyze the effect of fracture aperture on the microscopic characteristics of filled polyurethane grout. The experimental results demonstrate that, larger fracture apertures result in smaller maximum propagation distances and shorter propagation time. Besides, in fractures with a large aperture, a noticeable pressure drop occurs after grout gelation. The simulation results demonstrate that an increase in the aperture is inversely proportional to the polyurethane filling rate, whereas it is directly proportional to the cement filling rate. This disparity is attributed to different injection modes and driving force sources for these two grouting materials. In addition, the SEM test results show that larger fracture apertures correspond to greater microscopic pore diameters and fewer pore numbers in filled polyurethane grouts, which provides a microscopic explanation for the observed pressure drop during experimentation. The outcomes of this study offer valuable experimental and theoretical foundations for enhancing the understanding of self-expanding polyurethane grouting in rock fractures.