<p>Rare earth-based electromagnetic wave (EMW) absorbing materials have broad application prospects due to their strong dielectric loss and magnetic loss ability, but the effective enrichment and utilization of rare earth ions are still challenging. In this work, an anionic imidazolium-based metal-organic framework (MOF) was used to realize the enrichment strategy of multiple rare earth ions (La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Y³⁺). Thanks to its anionic coordination framework, metal-organic zeolite (MOZ)-200 can uniformly enrich these ions and convert them into highly dispersed high-entropy rare earth alloy nanoparticles <i>in situ</i> during the carbonization process. The obtained HE@C composites have a conductive and graphitized carbon matrix, rich multi-scale polarization centers, and heterogeneous interfaces, which effectively enhance the dipole polarization, interface polarization, and conductive loss. Therefore, the material achieves excellent electromagnetic wave absorption in the Ku band, with a minimum reflection loss of −79.20 dB and a maximum effective absorption bandwidth of 5.23 GHz. Further integrated with the polyurethane matrix, a multifunctional flexible electromagnetic wave absorption device was obtained, including electromagnetic wave absorption, photothermal heating, microwave de-icing, and hydrophobicity. This work provides a feasible strategy for the utilization of rare earth ions and promotes the design of flexible multifunctional EMW absorbing materials.</p>