Reducing　the　size　of　metal　nanoparticle　(NP)　cocatalyst　down　to　single-atom　level　to　improve　photocatalytic　efficiency　is　inevitably　accompanied　by　the　changes　of　its　coordination　environment,　geometric　configuration,　electronic　structure　and　active　site.　Thus,　the　construction　of　single　metal　atom　(SA)　photocatalyst　is　not　necessarily　a　panacea　for　activity　improvement　toward　target　catalytic　reactions.　Herein,　we　report　a　critical　and　benchmark　comparison　in　a　reasonable　framework　of　ZnIn2S4/Pt　NP　(ZIS/Pt　NP)　and　ZnIn2S4/Pt　SA　(ZIS/Pt　SA)　towards　photocatalytic　hydrogen　(H2)　evolution,　aiming　to　demonstrate　which　is　better　between　Pt　NP　and　Pt　SA　as　cocatalyst　in　boosting　photoredox　catalysis.　Mechanism　study　proves　that　the　higher　charge　separation/transfer　and　weaker　H*　adsorption　strength　over　ZIS/Pt　NP　than　ZIS/Pt　SA　promote　the　more　effective　reduction　of　protons　to　H2,　leading　to　the　higher　activity　of　ZIS/Pt　NP　than　ZIS/Pt　SA.　Our　work　is　expected　to　timely　inspire　the　critical　and　rational　thinking　on　the　function　and　intrinsic　mechanism　of　SA　and　NP　cocatalysts　in　enhancing　the　photoredox　catalysis　performance.　©　2024　Elsevier　B.V.