Transition metal catalysts based on proton-responsive ligands are crucial in the realm of chemical hydrogen storage. Nevertheless, the existence of multiple proton-responsive active sites on the ligand adds complexity to the mechanism, limiting our understanding of mechanistic preferences. In this study, density functional theory calculations are used to clarify the mechanistic preferences of iridium catalysts with multiple proton-responsive sites on the ligand for formic acid dehydrogenation. The calculated results show that, with the assistance of the α carbon atom (C2) of the pyrrole ligand site, the catalyst undergoes through an inner-sphere stepwise dehydrogenation mechanism. Moreover, the proton shuttle mechanism efficiently reduces the activation energy needed for H2 production, and solvent-assisted dehydrogenation shows lower energy barriers than substrate-assisted dehydrogenation. The reasons for mechanistic selectivity are further elucidated through Fukui function analysis, molecular plane parameter analysis, and the distortion-interaction model. These findings are anticipated to establish a theoretical foundation for the future design of high-performance catalysts for formic acid dehydrogenation.