Surface segregation constitutes an efficient approach to enhance the alkaline hydrogen evolution reaction (HER) activity of the bimetallic PtxNiy nanoalloys. Herein, we propose a new strategy by utilizing the small gas molecule of H2 as the structure directing agent (SDA) to in-situ induce Pt surface segregations over a series of PtNi5-n samples with extremely low Pt dopings (Pt/Ni = 0.2). Impressively, the sample of PtNi5-0.3 synthesized under 0.3 MPa H2 delivers an unprecedentedly low overpotential of 12.8 mV (-10 mA cm-2) and Tafel slop of 14.1 mV dec-1, which is superior than most of previously reported PtxNiy electrocatalyst. This is closely related to the strong in-situ inducing effect of H2 (0.3 MPa) leading to an obvious [email protected] core-shell nanostructure which can efficiently reduce the reaction Gibbs free energies. Finally, the specific mechanistic effects of H2 during PtNi5-n synthesis process are well illustrated based on the combined experimental and theoretical studies.