CO2 hydrogenation is full of challenge because both H2 and CO2 are activated at the same time.This reaction also has 100% at. economy.Most of the reported catalysts for CO2 hydrogenation are based on noble metal.To find out more effective iron based catalysts for the CO2 hydrogenation to formic acid, totally, the reaction processes catalyzed by 12 different PNP-Fe (PNP=2, 6-bis (di-tert-butylphosphinomethyl) pyridine) compounds were investigated.The theoretic calculation results revealed that the CO2 hydrogenation included two steps:H2 activation and CO2 inserting into the metal-hydride bond.The H2 activation is the rate-determining step.It was found that P atom substitution on the pyridine ring could obviously reduce the H2 activation barrier.Based on these findings, an effective Fe catalyst was designed, whose H2 activation barrier was only 85.6 kJ/mol, being comparable to the data of precious metal catalyst.The H2 activation barriers range from 85.6 to 126.4 kJ/mol for different Fe-based catalysts investigated here, indicating that the modification of ligand has great influence on the catalytic reactivity for the CO2 hydrogenation.