Abstract: To address the compatibility and interfacial modulation of reactive ionic liquids (ILs) in aqueous polyurethane (WPU) chemical environments, L-malic acid/triethylamine IL was introduced to construct composite sizing systems with WPU∶IL solid mass ratios of 1∶0.4, 1∶0.8, and 1∶1.2. The effects of IL content on carbon fiber (CF) surface coating morphology, flexural properties, interlaminar shear strength (ILSS), interfacial shear strength (IFSS), and interfacial failure behavior of CF/epoxy (EP) composites were systematically investigated. Results indicate that the interfacial performance of the composites initially increases and then decreases with increasing IL content. At a WPU∶IL ratio of 1∶0.8, a relatively continuous and uniform coating layer forms on the CF surface, achieving ILSS, IFSS, and flexural strength values of 75.0 MPa, 104.6 MPa, and 1681 MPa for CF/EP, respectively—representing improvements of 102.7%, 94.4%, and 52.1% compared to unsized CF/EP, and all exceeding those of commercially sized CF composites. Fracture surface and single-fiber debonding morphologies reveal that this composition effectively suppresses interfacial delamination, shifting the failure mode toward a mixed interface/matrix mode. Model reactions, FTIR, and LF-NMR analyses demonstrate that the IL can react with EP and influence its network formation. The study shows that the WPU/IL system exhibits significant composition dependence, with 1∶0.8 being the optimal ratio among those examined.