Electrocatalytic reduction of CO2 into value-added chemicals has been considered as a promising pathway to mitigate the energy crisis and global warming. Iron porphyrins have been extensively studied for electrocatalytic CO2 reduction reaction (CO2RR), known for their ability to promote CO2-to-CO conversion. However, the mechanism of CO2-to-HCOO− conversion by Fe porphyrin remains unclear. Here, by means of density functional theory (DFT) calculations, we investigated the detailed mechanism of a novel Fe porphyrin catalyst for CO2 reduction to HCOO− in its Fe(I) state. Our results demonstrated that the reduction of CO2 to HCOO− proceeds via the C-protonation of an FeII-OCO•− complex, rather than through the hydrolysis of an FeIII-COOH complex or CO2 insertion in an Fe−H bond. Furthermore, the FeIII-COOH complex is found to be a unstable intermediate. Protonation of its hydroxyl group, accompanied by C−OH bond cleavage to produce CO, is both thermodynamically and kinetically unfeasible. Instead, the FeIII-COOH complex undergoes a coordination switch followed by a conformational change to form the active FeII-OCO•− complex, which promotes the production of HCOO−. Moreover, the single-electron reduction of FeIII-COOH gives FeII-COOH, leading to formation of CO rather than HCOO−. The insights gained from this study may contribute to design of electrocatalysts for selective CO2 reduction to formate.