Multifunctional polyurethane (PU) holds significant promise as a next-generation smart material, yet its inherent flammability limits widespread application. Currently, research on condensed-phase flame retardancy primarily focuses on promoting char layer formation, while the critical affecting factor of char layer structural stability is often overlooked. In this study, inspired by the sand-fixation mechanism, a cluster-structured flame retardant was designed using rare earth metals as the central core and phosphoric acids as functional end groups to construct a root-like framework. This biomimetic flame-retardant strategy successfully achieved dual functionality: catalytic carbonization and structural support. Compared to the control sample, the heat release rate (HRR) and smoke production rate (SPR) of ATMP–Eu-co-PU were decreased by 55% and 60%, respectively. The residual char morphology after combustion resembles a distinctive “skeleton–sphere” composite char layer structure. Additionally, the gamma-ray shielding efficiency of (P–Eu)₇-co-PU increased by 10.6%, while its UV shielding efficiency reached 99%. The synthesized polyurethanes can also be utilized for visual UV monitoring, coating defect detection, and cryptographic applications.