Herein, magnetic g-C3N4/g-carbon foams (CNFs) are fabricated via impregnating the as-prepared melamine-formaldehyde (MF) foams in iron acetylacetone (Fe(acac)(3)) N,N-dimethylformamide (DMF) solutions with different concentrations followed by in situ pyrolysis. The corresponding electromagnetic wave (EMW) absorption behaviors are investigated. CNFs display solid 3D foam architectures with narrow and smooth skeletons, which consist of graphitic C3N4/carbon and CFe15.1. The in situ assynthesized alpha-Fe, Fe3O4, and Fe3C particles are dispersed on the skeleton surface. With the increase of Fe elemental content, the EMW absorption performance of CNFs is enhanced. Among them, the minimum reflection loss value of CNF-3 is as low as -52.17 dB, with a matching thickness (t(m)) of 4.2S mm and a broad effective absorbing bandwidth of 9.37 GHz. Besides, CNF-3 exhibits potential simultaneously effective absorption performance from the X-band to the Ku-band. This superior absorption behavior is attributed to Maxwell-Wagner-Sillars polarization, residual loss effect, enriched interface polarization effect, and macroporous structures. Further research studies demonstrate the potential application of CNF-3 as a reinforcing agent for PCL-based 3D printing EMW absorbers with improved mechanical properties. The corresponding EMW absorption matching frequency can be adjusted by the number of 3D printing layers.