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 H 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.