The efficiency of zero-valent iron (Fe-0)for the degradation of contaminants in water or soil can be highly reduced by side reactions with oxygen or water. This work was conducted to test whether this drawback can be effectively suppressed by the carbonation of Fe-0 with pyrolyzed biomass, which forms a Fe3C composite. The composite Fe3C was characterized and its reactivity and stability were assessed in batch tests with methyl orange (MO) as a model pollutant. The results indicated that the removal rate of MO on Fe3C composite was higher than that of Fe-0 (7.587 mg/(g.min) vs. 4.306 mg/(g.min)) at pH 4, where the degradation mechanism was confirmed by high-performance liquid chromatography-mass spectrometry. More importantly, the produced iron oxide in the Fe3C composite was highly suppressed. Regeneration studies showed that after three times of cycling, the removal efficiency of MO on Fe3C composite was kept to 99.42%, but Fe-0 almost lost its reactivity. In situ chemical reduction of a colorimetric redox probe (indigo-5, 5'-disulfonate, I2S) quantitatively demonstrated that Fe3C composite has the reduction kinetics of I2S obviously slower than Fe-0, indicating that Fe3C composite improved the stability of incorporated Fe-0 to resist the side oxidation.