Abstract: To realize the application of electromagnetic wave absorption (EWA) devices in humid marine environments, bifunctional EWA materials with better EWA capacities and anticorrosion properties have great exploration significance and systematic research requirements. By utilizing the low-cost and excellent magnetic and stable chemical characteristics of barium ferrite (BaFe₁₂O₁₉) and using the high dielectric loss and excellent chemical inertia of nanocarbon clusters, a new type of nanocomposites with carbon nanoclusters encapsulating BaFe₁₂O₁₉ was designed and synthesized by combining an impregnation method and a high-temperature calcination strategy. Furthermore, Ce–Mn ions were introduced into the BaFe₁₂O₁₉ lattice to improve the dielectric and magnetic properties of BaFe₁₂O₁₉ cores significantly, and the energy band structure of the doped lattice and the orders of Ce replacing Fe sites were calculated. Benefiting from Ce–Mn ion doping and carbon nanocluster encapsulation, the composite material exhibited excellent dual functionality of corrosion resistance and EWA. When BaCe_0.2Mn_0.3Fe_11.5O₁₉-C (BCM-C) was calcined at 600°C, the minimum reflection loss of −20.1 dB was achieved at 14.43 GHz. The Ku band’s effective absorption bandwidth of 4.25 GHz was achieved at an absorber thickness of only 1.3 mm. The BCM-C/polydimethylsiloxane coating had excellent corrosion resistance in the simulated marine environment (3.5wt% NaCl solution). The \left|\left.Z\right|\right._0.01\mathrm \mathrmH\mathrmz value of BCM-C remained at 10⁶ Ω·cm² after 12 soaking days. The successful preparation of the BaFe₁₂O₁₉ composite encapsulated with carbon nanoclusters provides new insights into the preparation of multifunctional absorbent materials and the fabrication of absorbent devices applied in humid marine environments in the future.