Polyurethane grouting serves as an important solution for both anti-seepage and reinforcement in underground construction projects. A comprehensive understanding of the propagation characteristics of polyurethane grout is essential for effective grouting design, particularly in the presence of flowing water and connected fracture systems. This study aims to investigate the propagation behaviors and sealing efficiency of polyurethane grout within connected fractures under flowing water conditions. A series of laboratory experiments were conducted employing a self-designed laboratory grouting device for connected fractures. The results demonstrate that the propagation patterns vary significantly with different fracture connectivity configurations. Dynamic water-grout interactions exhibit three distinct temporal stages that directly influence propagation dynamics: injection, expansion, and equilibrium phases. The propagation width is predominately established during injection, while the interaction between polyurethane grout and water is achieved through the expansion stage. The influence of initial water flow rate and fracture connecting pattern on the propagation and sealing efficiency of polyurethane grout was analyzed. The results show that higher initial water flow rates decrease the upstream propagation distance and width while altering the pressure distribution within fractures, decreasing pressure in the primary fracture but increasing it in the secondary fractures. This hydraulic response is quantitatively characterized by an exponential decay relationship between grout shear stress and initial water flow rate. The study reveals that fracture intersections play a critical role in grout accumulation and sealing enhancement. Notably, vertically connected fractures demonstrate superior and more stable sealing efficiency compared to horizontally-vertically connected fractures with significantly reduced sensitivity to flow rate variations. Additionally, viscosity and flow rate ratios between polyurethane grout and water are positively correlated with sealing efficiency, with viscosity ratios playing a more dominant role than injection rates. These findings provide valuable insights for optimizing polyurethane grouting performance in connected fracture systems under flowing water conditions, offering practical guidance for underground engineering applications.