In this work, the surface properties of triazine and heptazine based g-C3N4 (001) are distinguished to understand the basic function of each framework in the catalytic reduction towards CO2 to CO via dispersion corrected density functional theory (DFT-D). It is found that heptazine based g-C3N4 (h-g-C3N4) (001) has narrower band gap and more delocalized electron distribution compared to triazine based g-C3N4 (t-g-C3N4) surface, which lead to enhanced visible-light absorption and electron mobility that favor its catalytic performance. Moreover, h-gC(3)N(4) (001) exhibits much lower energy barriers along the two possible reaction pathways, in which the COOH-mediated mechanism dominates the reaction paths. After Ni loading, the surface properties are greatly modified on g-C3N4 (001). The energy barrier of CO2 conversion is significantly reduced that leads to enhanced reduction activity, and the intermediate COOH is the preferred product on Ni loaded t-g-C3N4 and h-g-C3N4 surfaces. Considering the surface properties and reaction activity, Ni loaded h-g-C3N4 is identified as a promising efficient catalyst. It is concluded that h-g-C3N4 has preferable surface properties that promote the catalytic performance and is recommended as the prior substrate material in the pursuit of novel composites for CO(2 )conversion. These findings could provide useful guidance in designing efficient g-C3N4 based catalysts and offer deep insights into the CO2 reduction mechanisms.