As an extension of single-atom catalysts, despite the increased opportunities to optimize the hydrogen evolution reaction (HER) activity with the variation of the composition, dual-metal-atom catalysts, i.e., dimers, are deeply trapped in a design blind spot due to the lack of the essential recognition of the intrinsic catalytic mechanism at the atomic level. Herein, based on first-principles calculations, a series of platinum-transition metal dimers were constructed on nitrogen-doped graphene (PtM-NDG, M = Fe, Co, Ni, Cu) to reveal the effects of the internal (i.e., M atom) and external (i.e., NDG substrate) environments on the HER activity. Computational results show that the original over-adsorption of hydrogen intermediate (H*) of PtM dimer is weakened after the introduction of NDG, and the optimal active site migrates from the Pt in PtM dimer to the Pt-M bridge in PtM-NDG, triggered by the redistribution of the charge density of the metal atoms. In particular, the M atom switches from tuning the d-band center of the Pt atom to indirectly assist the adsorption behavior of Pt in the PtM dimer to the direct participation in the bonding with H* in PtM-NDG via its own d-band to regulate the distribution of σ and σ*, which enables fine modulation of the bond strength with H*. Moreover, the overall hydrogen evolution performance of PtM-NDG is mainly determined by the d-band center of the M atom. Furthermore, PtFe-NDG with the lowest energy barrier of the rate-determining step stands out in the process of H2 desorption and water dissociation. The present work deepens our understanding of the effects of the metal dopant and substrate on the catalytic performance of platinum.