Phosphides　exhibit　relatively　low　overpotential　for　electrical　hydrogen　evolution　reaction　(HER),　thus　they　have　great　potential　to　be　used　for　cocatalyst　for　photocatalyst.　Cu3P,　as　a　p-type　semiconductor,　tends　to　form　a　p-n　junction　with　an　n-type　photocatalyst.　Typically,　it　is　treated　as　a　sensitizer　to　extend　the　light　absorption.　However,　its　function　and　work　mechanism　are　not　fully　understood　in　the　catalyst　system.　In　this　report,　we　synthesized　g-C3N4　and　loaded　Cu3P　nanoparticle　on　its　surface.　The　photoluminescence　(PL)　spectra,　photocurrent　and　electrochemical　impedance　spectra　confirm　the　Cu3P　greatly　enhance　the　charge　separation　process.　Electrochemical　HER　results　indicate　that　the　composites　have　lower　over-potential　for　HER.　These　results　confirm　the　Cu3P　works　as　a　cocatalyst　in　the　system,　not　a　sensitizer.　Further,　we　tracked　the　photogenerated　electron　transfer　direction　via　photodeposition　of　Pt　nanoparticles.　The　Pt　nanoparticles　tend　to　deposit　near　the　Cu3P　nanoparticles.　That　illustrates　the　photogenerated　electron　will　be　left　on　Cu3P　nanoparticles.　On　the　other　hand,　the　photocatalytic　decomposition　of　Rhodamine　B　(RhB)　illustrates　that　the　holes　are　left　on　the　g-C3N4　due　to　both　g-C3N4　and　Cu3P/g-C3N4　have　similar　decomposition　rate,　but　the　Cu3P　cannot　decompose　RhB.　Based　on　these,　we　proposed　the　photogenerated　electron　of　g-C3N4　recombine　with　the　hole　of　Cu3P,　the　photogenerated　electron　of　Cu3P　will　be　left　for　HER.　That　reasonably　explain　the　cocatalyst　function　of　Cu3P　in　the　composite　catalyst　system.