A unique synthesis strategy that combines grinding and microwave-assisted heating was proposed to expediently fabricate iron nanoparticles supported by g-C3N4. The resulting catalyst exhibits high-purity Fe5C2 with a core-shell structure during the reaction process. Under relevant industrial conditions (320 °C, 2.0 MPa), it achieves a C5+ selectivity of 52.1% at a CO2 conversion of 27.5%.