Proton exchange membrane water electrolysis (PEMWE) coupled with renewable energy is regarded as a sustainable approach for green hydrogen production. However, the low reserve and high cost of platinum cathode electrocatalysts greatly limit its widespread deployment. Herein, we report a phase conversion strategy to obtain an efficient low-Pt loading electrocatalyst composed of multi-atomic Pt substituted 1T-MoS2 nanosheet grown on a cross-interlocking porous carbon matrix (Pt@1T-MoS2/C). In combination with experimental and theoretical results, the 2H-to-1T phase conversion of MoS2 induced by the Pt substitution is investigated, and the as-formed Pt-S-Mo active site accounts for the excellent hydrogen evolution reaction (HER) performance. Benefiting from the improved electron transfer efficiency and Pt-S-Mo active sites, the Pt@1T-MoS2/C delivers a high mass HER activity of ∼58.9 A·mgPt−1, and an excellent stability of 200 h at 1000 mA∙cm−2 in a practical PEMWE device.