Graphitic carbon nitride (g-C3N4) is recognized as a favorable substrate for monoatom catalysts due to its uniform nanoholes for anchoring metal monoatoms, while the oxygen evolution reaction (OER) overpotential (ηOER) values of g-C3N4-based metal monoatom catalysts are still large. To reduce the ηOER values, a class of novel TM1NM1NM1/g-C3N4 was designed via density functional theory simulations, where TM1 = Fe1, Co1 or Ni1 and NM1 = C1, N1 or O1. Contributing by two extra-NM1 atoms, the OER catalytic activities of these materials were effectively improved owing to the shortened TM1-NM bonds and weakened chemical activity of TM1 atoms. Based on the volcano activity relationship between the theoretical overpotential (ηOER) and d band center of the TM1 atom (εd), the chemical activity of TM1 atoms needs to be adjusted to a suitable magnitude (εd near -4.883 eV) for a good catalytic activity. The designed Fe1C1O1/g-C3N4 with the εd of -4.893 eV exhibited an excellent OER catalytic activity of ηOER = 0.219 V. This strategy was applied to devise the reaction active sites and highly efficient catalysts by adjusting the chemical activity of the TM1 atom with suitable extra-NM1 atoms.