Molybdenum disulphide (MoS2) is proven to be a promising catalyst for hydrogen evolution reaction (HER), but the HER performance of reported MoS2-based catalysts is still limited by its poor conductivity and low density of active sites. Herein, a Pt-doped MoS2 (Pt@MoS2) catalyst is synthesized by a potential-cycling method, which introduces the Pt dopant into the MoS2 lattice and achieves partial 2H to 1T phase conversion of MoS2 simultaneously. Benefitting from the optimized geometric and electronic structure of MoS2, the Pt@MoS2 exhibits a low overpotential of 88.43 mV at 10 mA cm���2, which is decreased by two-thirds as compared to that of the pristine MoS2. A comprehensive study reveals the position and the contribution of Pt atom in electronic structure modulation of MoS2. Theoretical calculations further reveal that the S atom adjacent to the Pt in MoS2 acts as the most active site for HER, and possesses a small hydrogen adsorption free energy (���GH*) of ~ 0.04 eV, similar to the benchmark Pt catalyst. This study opens up a new avenue for designing MoS2 and other transition metal dichalcogenide-based electrocatalysts with enhanced HER performance, as well as providing in-depth understanding on the HER mechanism in external metal-activated MoS2 catalyst.