Intense　researches　for　pursuing　effective　catalysts　for　alkaline　hydrogen　evolution　reaction　(HER)　have　been　carried　out.　Particularly,　Pt-based　materials　having　heterostructures　have　shown　excellent　electrocatalytic　performance.　However,　there　are　only　few　investigations　on　the　transformation　from　an　alloy　to　a　heterostructure　and　the　main　reason　for　superior　activity.　Herein,　we　demonstrated　that　Pt-Ni　heterostructures　could　be　formed　by　only　introducing　NaOH　in　the　synthesis　of　Pt-Ni　alloys.　Density　functional　theory　calculations　revealed　that　the　added　OH-　can　influence　the　energy　barriers　of　atomic　reduction　and　migration,　thus　driving　the　formation　of　a　heterostructure　with　atomic　segregation.　Moreover,　it　was　proved　that　the　enhanced　HER　performance　is　attributed　to　favorable　H　and　OH　energetics　arising　from　the　atomic　segregation.　Acid　treatment　emphasized　the　key　factors　in　determining　the　activity.　This　work　proves　that　adding　OH-　can　lead　to　the　formation　of　nanocatalysts　with　atomic　segregation,　and　tuning　the　atomic　segregation　of　Pt-Ni　catalysts　is　an　effective　way　to　optimize　their　H　and　OH　adsorption　abilities,　thus　enhancing　the　alkaline　HER　performance.