The invention discloses a preparation method of a catalyst for photo-catalytically splitting water to produce hydrogen and relates to nano materials. The preparation method includes: placing urea into a ceramic crucible with a cover, and calcining in a muffle furnace to obtain yellow g-C3N4 polymer material; under nitrogen protection, ultrasonically dispersing the g-C3N4 polymer material into tetrahydrofuran solution, using lithium metal as the electron donor and naphthalene as the first electron acceptor, performing solution-phase stripping in the presence of halogenated hydrocarbon, adding ethanol into the reaction product after reaction to remove unreacted lithium metal, centrifuging, washing the obtained solid product, and drying to obtain g-C3N4 nano-plates; ultrasonically dispersing the g-C3N4 nano-plates into water, adding graphene oxide, continuing ultrasonic dispersion to obtain a mixed solution, transferring the mixed solution into a reaction kettle, rising temperature to 140-200 DEG C, keeping the temperature for 2-12 hours, then cooling to room temperature, performing suction filtration to obtain solid product, and performing vacuum drying to obtain the catalyst for photo-catalytically splitting water to produce hydrogen.