Electrocatalytic reduction of CO2 to produce high value-added products is an important way to achieve sustainable production of green energy, but it is limited by high complexity of the catalyst structure and reaction process, making the rational design of the catalyst and the targeted generation of specific products a huge challenge. Herein, based on the penta-octa-graphene (POG) with sp2 and sp3 hybrid as the prototype, we theoretically guide the design of CO2 electrocatalytic reduction (CO2RR) catalyst over pure POG and TM doping POG (TM@POG). Our mechanistic study highlights two key factors in new pathway for CH4 production and catalytic performance, i.e., TM provides electron transfer for the key intermediate *CHO, promoting the activation of CO2, and the type of protonation for CO2RR is determined by the charge accumulation on the O atom of the intermediates. This reduces the complexity of the reaction and provides a predictive effect on the formation of intermediates. More importantly, the proper values of the Gibbs free energy indicate that TM@POG is an ideal catalyst candidate, especially the Fe@POG, which is superior to Cu (211) material. On this basis, three simple descriptors (Bader charge, d-band center εd and descriptor φ) based on the inherent properties of atoms were constructed to reveal the underlying causes of the improvement of catalytic activity. Our results provide a new understanding of the ’structure-performance’ relationship of carbon-based electrocatalysts, thus providing a useful strategy for the design of catalysts for CO2RR.