The rapid development of communication technologies has intensified electromagnetic pollution, creating an urgent demand for high-performance microwave absorbers. Herein, Fe@CNTs composites were synthesized via molten salt electrochemical reduction using CO₂ as the carbon source in a Na₂CO₃–K₂CO₃–CaCO₃ electrolyte. The microstructure, electromagnetic parameters, and microwave absorption performance were systematically investigated. The as-prepared sample exhibits an effective absorption bandwidth of 5.1 GHz at a thin thickness of 1.7 mm, surpassing most reported CNTs-based absorbers. Furthermore, a trapezoidal pyramid metastructure was designed to further broaden the bandwidth; the optimized metastructure achieves full-band absorption covering 2–18 GHz at a total thickness of 16.4 mm, representing a three-fold bandwidth enhancement over the planar configuration. This work provides a green, scalable synthesis route and an effective structural engineering strategy for ultra-broadband microwave absorption .