Abstract: Surface decorated blockboard with paper impregnated thermosetting resins, as a type of wood-based panel used for indoor furniture decoration, is prone to surface cracking due to the poor dimensional stability of its substrate and weak interfacial bonding strength with the impregnated paper. To address this issue, this study synthesized an amino-terminated hyperbranched polymer (HBP-NH₂) and compounded it with a waterborne isocyanate to prepare a composite modifier (MD-HBP-NH₂), investigating its mechanism for interfacial reinforcement between the impregnated paper and blockboard. The results show that: (1) After mixing the composite modifier with melamine-formaldehyde (MF) impregnation resin, it had no significant effect on the curing time of the impregnation adhesive. The modified impregnated paper maintained a tensile strength of 6.11 kN/m, while its elongation increased to 0.62%. (2) Applying the composite modifier to the blockboard surface resulted in a water contact angle of 94.16°, surface free energy close to that of the hot-pressed impregnated paper, a 52.3% significant reduction in dimensional change rate under dry-heat conditions compared to natural wood, and an increased bonding strength of the decorated blockboard to 1.24 MPa. Both cold-hot cycle resistance and crack resistance reached grade 5. (3) Interfacial analysis revealed that the composite modifier penetrated the impregnated paper and the wood surface of the blockboard through physicochemical bonding, forming a "root-like" reinforcement network. This significantly enhanced the interfacial bonding strength between the impregnated paper and the blockboard. Through molecular modification design and interfacial reinforcement, this study significantly improved the interfacial bonding strength, water resistance, and dimensional stability of surface decorated blockboard with paper impregnated thermosetting resins, effectively preventing surface cracking phenomena.