The decoration by transition-metal single atoms shows great potential in enhancing the photocatalytic performance of semiconductors. Nevertheless, the factors accounting for this enhancement remain unknown. Herein, single atoms (Fe, Co, Ni, Cu, Zn) loaded g-C3N4 were used as prototypes to investigate the mechanism through density functional theory (DFT). The results indicate that Fe, Co and Ni single atoms are inclined to penetrate into g-C3N4, prolonging the charge carrier migration distance. Furthermore, these single atoms can act as bridge to promote charge carrier transfer across g-C3N4 interlamination through the reconstitution of molecular orbitals. Among these five metals, built-in electric field is created in Fe/g-C3N4 and Co/g-C3N4 models, effectively suppressing the recombination of electron-hole pairs. In contrast, Cu and Zn atoms can only be loaded on the g-C3N4 surface, contributing trivial to the interlayer charge carrier migration. Our work provides theoretical support for the study of transition-metal single atom/g-C3N4 systems and gives new insight into the photocatalytic performance enhancement.