Single-atom Fe-N-C catalysts has attracted widespread attentions in the oxygen reduction reaction (ORR). However, the origin of ORR activity on Fe-N-C catalysts is still unclear, which hinder the further improvement of Fe-N-C catalysts. Herein, we provide a model to understand the ORR activity of Fe-N4 site from the spatial structure and energy level of the frontier orbitals by density functional theory calculations. Taking the regulation of divacancy defects on Fe-N4 site ORR activity as examples, we demonstrate that the hybridization between Fe 3dz2, 3dyz (3dxz) and O2 π* orbitals is the origin of Fe-N4 ORR activity. We found that the Fe-O bond length, the d-band center gap of spin states, the magnetic moment of Fe site and *O2 as descriptors can accurately predict the ORR activity of Fe-N4 site. Furthermore, these descriptors and ORR activity of Fe-N4 site are mainly distributed in two regions with obvious difference, which greatly relate to the height of Fe 3d projected orbital in the Z direction. This work provides a new insight into the ORR activity of single-atom M-N-C catalysts.