The ethylene polymerization mechanism of the 2-methyl-8-(benzimidazol) quinolyliron(II) pre-catalyst is investigated by the DFT calculations, illustrating the possible intermediates with their geometrical and spin configurations. Regarding either methyl or ethyl group bonding on iron cores, the energy barriers for ethylene insertion have been extensively calculated. Within the iron‑methyl species, both resting state and transition state favor the configurations at high-spin state (quintet); whilst the iron-ethyl species prefer the low-spin state. According to the energy barriers, the chain propagation is more favorable than chain transfer for the bidentate iron pre-catalyst, which is well consistent with the experimental observation.