Platinum (Pt)-based electrocatalysts remain the only practical cathode catalysts for proton exchange membrane water electrolysis (PEMWE), due to their excellent catalytic activity for acidic hydrogen evolution reaction (HER), but are greatly limited by their low reserves and high cost. Here, we report an interfacial engineering strategy to obtain a promising low-Pt loading catalyst with atomically Pt-doped molybdenum carbide quantum dots decorated on conductive porous carbon (Pt-MoC_x@C) for high-rate and stable HER in PEMWE. Benefiting from the strong interfacial interaction between Pt atoms and the ultra-small MoC_x quantum dots substrate, the Pt-MoC_x catalyst exhibits a high mass activity of 8.00 A·mg_Pt⁻¹, 5.6 times higher than that of commercial 20 wt.% Pt/C catalyst. Moreover, the strong interfacial coupling of Pt and MoC_x substrate greatly improves the HER stability of the Pt-MoC_x catalyst. Density functional theory studies further confirm the strong metal-support interaction on Pt-MoC_x, the critical role of MoC_x substrate in the stabilization of surface Pt atoms, as well as activation of MoC_x substrate by Pt atoms for improving HER durability and activity. The optimized Pt-MoC_x@C catalyst demonstrates > 2000 h stability under a water-splitting current of 1000 mA·cm⁻² when applied to the cathode of a PEM water electrolyzer, suggesting the potential for practical applications.