The　electrochemical　hydrogen　evolution　reaction　(HER)　has　become　one　of　effective　ways　to　produce　hydrogen.　MoS2　is　a　promising　electrocatalyst　for　the　HER,　and　loading　noble　metal　atoms　on　MoS2　support　can　further　enhance　its　HER　activity.　In　present　work,　the　CO　was　employed　as　a　structure-directing　agent　to　in　situ　synthesize　MoS2-n　(n　=　0,　0.5,　1.0,　1.4　and　2.0　MPa,　representing　the　CO　pressure).　Among　them,　the　MoS2-1.4　with　highest　amounts　of　sulfur　vacancies　and　superior　alkaline　HER　performance　was　selected　as　the　support　for　Ru　loadings.　Sulfur　vacancies　can　stabilize　Ru　nanoparticles,　prevent　nanoparticles　from　aggregating,　and　facilitate　the　formation　of　small-sized　nanoparticles.　By　optimizing　the　loading　amounts　of　Ru,　the　5Ru@MoS2-1.4　with　the　Ru　loading　of　5　wt.%　and　nanoparticle　size　of　2　nm　exhibits　the　best　alkaline　HER　activity　by　displaying　a　low　overpotential　of　55　mV　and　Tafel　slope　of　68.6　mV　dec−1　at　a　current　density　of　10　mA　cm−2　in　1.0　M　KOH　medium.　This　can　be　majorly　related　to　the　synergistic　effect　of　MoS2-1.4　support　and　small-sized　Ru　nanoparticles,　wherein　the　CO　plays　a　significant　role　in　regulating　the　structure　of　MoS2-1.4　support　that　favors　dispersion　of　Ru　nanoparticles.　Generally,　present　work　provides　a　strategy　to　enhance　the　HER　catalytic　performance　of　MoS2,　which　is　favorable　for　other　highly-efficient　catalyst　design.　©　2024