Exploring　highly　active　and　stable　electrocatalyst　using　trace　noble　metals　for　hydrogen　evolution　reaction　(HER)　is　urgently　needed　but　still　challenging.　Here,　atomically　dispersed　Pt　on　ultrafine　Ru　nanoclusters　(similar　to　1.46　nm)　loaded　by　the　commercial　acetylene　black　(Acet)　support　is　fabricated　by　a　facile　one-step　cold-plasma　technique.　The　Pt　single　atoms　on　Ru　nanoclusters　electrocatalyst　(Pt-0.47-Ru/Acet)　with　0.47　wt%　content　of　Pt　exhibit　an　excellent　HER　activity　in　all　pH,　achieving　ultralow　overpotential　of　only　17,　28,　and　8　mV　at　10　mA.cm(-2)　in　1　M　KOH,　0.5　M　H2SO4,　and　1　M　PBS,　respectively.　And　the　mass　activity　of　Pt-0.47-Ru/Acet　catalyst　in　alkaline　is　about　5.54　and　2.15　times　that　of　commercial　Ru/C　and　Pt/C　catalyst　at　an　overpotential　of　100　mV,　respectively.　Meanwhile,　after　8000　CV　cycles,　there　is　barely　performance　decrease　for　Pt-0.47-Ru/Acet,　displaying　good　stability.　DFT　calculation　reveals　that　the　Pt　single　atoms　could　effectively　regulate　the　electronic　structure　of　Ru　clusters,　and　reduce　the　energy　barriers　of　water　dissociation　(Volmer　step)　as　well　as　subsequent　hydrogen　evolution　(Heyrovsky　step),　thus　enhancing　the　alkaline　HER　performance　effectively.　This　work　provides　an　effective　approach　to　designing　a　high-performance　HER　electrocatalyst　with　the　atomic-dimension　Pt　surface　modification.