MoS2　has　drawn　great　attention　as　a　promising　Pt-substituting　catalyst　for　the　hydrogen　evolution　reaction　(HER).　This　work　utilizes　H-2　as　the　structure　directing　agent　(SDA)　to　in　situ　synthesize　a　range　of　Co-MoS2-n　(n　=　0,　0.5,　1.0,　1.4,　2.0)　with　expanded　interlayer　spacings　(d　=　9.2　-　11.1　angstrom),　which　significantly　boost　their　HER　activities.　The　Co-MoS2-1.4　with　an　interlayer　spacing　of　10.3　angstrom　presents　an　extremely　low　overpotential　of　56　mV　(at　10　mA　cm(-2))　and　a　Tafel　slope　of　32　mV　dec(-1),　which　is　superior　than　most　reported　MoS2-based　catalysts.　Density　function　theory　calculations　are　used　to　gain　insights　that　i)　the　H-2　can　be　dissociatively　adsorbed　on　MoS2　and　greatly　affect　the　related　surface　free　energy　by　regulating　the　interlayer　spacing;　ii)　the　expanded　interlayer　spacing　can　significantly　decrease　the　absolute　value　of　Delta　G(H),　thereby　leading　to　greatly　promoted　HER　activity.　Additionally,　the　large　amounts　of　1T　phase　(73.9-79.2%)　and　Co-Mo-S　active　sites　(40.9-91.3%)　also　contribute　to　the　enhanced　HER　activity　of　the　synthesized　samples.　Overall,　a　simple　new　strategy　for　in　situ　synthesis　of　Co-MoS2　with　an　expanded　interlayer　spacing　is　proposed,　which　sheds　light　on　other　2D　energy　material　designs.