<p>Indigo-dyed denim is widely used in clothing because of its fashionable appeal and durability. However, this fabric exhibits poor color fastness, especially wet rubbing and washing fastness. Post-treatment of indigo-dyed denim with high molecular weight fixing agents is an effective method to enhance its color fastness. In this study, a series of quaternized epoxidezed glycidyl ether-modified polyurethane fixing agents with quaternary ammonium cation and multiple hydroxyl groups were synthesized by adjusting the molar ratio of glycidyl ether (AGE) to polyurethane terminal agent hydroxyethyl acrylate (HEA). The molecular weight, structure, and thermal stability of the obtained products were investigated using GPC, FTIR, H¹NMR, and TGA. These fixing agents were then applied to indigo-dyed denim, and the finished fabric was characterized using XPS, SEM, and zeta potential analysis. The results showed that the molecular weight of the fixing agent gradually increased as the <i>n</i>(AGE)/<i>n</i>(HEA) ratio decreased, with a number-average molecular weight ranging from approximately 10,000–20,000. IR spectra indicated the presence of C–N⁺ stretching vibration absorption peaks near 930–960&#xa0;cm⁻¹. The color shade of the indigo-dyed denim remained unchanged after finishing with the fixing agent. Furthermore, its dry and wet rubbing fastness improved from rating 2 and 1 to rating 4 and 2–3, respectively. The washing fastness also increased to rating 4 to 4–5. The whole fastness performances are comparable to or 0.5 ~ 1 ratings surpasses that of the commercial fixing agents CPFA-1 and CPFA-2. XPS and SEM analyses of the treated indigo-dyed denim revealed that the fixing agent adhered to the fiber surface and formed a robust bond with the fiber. This phenomenon is likely attributed to the formation of a hydrogen-bonded cross-link network among the fixing agent, indigo dye molecules and denim fibers. Consequently, this interaction significantly enhanced the binding of the indigo dye to the fiber.</p>