<p>The preservation of Ming-Qing lacquer furniture requires compatible backfill materials to mitigate cracking and moisture damage, which are exacerbated by incompatible synthetic polymers. This study introduces an innovative strategy by integrating traditional fish glue with advanced analytical techniques, including orthogonal experiments and thermogravimetric-differential scanning calorimetry (TG-DSC). Thermal-mechanical characterization revealed that 15 wt% fish glue optimally reinforced lacquer-ash composites. Under optimized curing conditions (40 °C, 70% RH, 4 h, and 8 g/m²), the composite achieved a dry adhesive strength of 1.61 MPa and a waterproof rate of 85.32%, outperforming commercial epoxy-based fillers. Mechanical analysis confirmed that the high thermal stability (148 °C decomposition temperature) and low water absorption (6.0% after 7 days) of fish glue suppressed interfacial degradation. Compared with egg white and glutinous rice paste, the composite demonstrated superior hydrophobicity and cohesive strength. This research bridges traditional craftsmanship with modern materials science, offering a sustainable solution to heritage preservation.</p>