The reaction mechanism of CO2 hydrogenation catalyzed by [FeH(PP3)]BF4 (PP3 = P(CH2CH2PPh2)3) had been investigated by DFT calculations. Our calculations indicated that the reduction of carbon dioxide could be carried out via two spin states, the high-spin (HS) triplet state and the low-spin (LS) singlet state. The minimum energy crossing points (MECPs) on the seam of two intersecting PESs (potential energy surfaces) were searched out. Some interesting phenomena, such as the open-loop phenomenon, and the O-rebound process, were demonstrated to be the important causes of the spin crossover. All these calculations gave us insight into the essence of the related experiment from the macro point of view, and helped to verify which spin states the related complexes pertinent were in. All of these researches would help advance the development of efficient and structurally tailorable CO2 hydrogenation catalysts.