The invention relates to I-III-VI2 group metal sulfide, in particular to a method for preparing molybdyl modified CuInZnS quantum dot photocatalyst from copper acetate, zinc acetate, indium nitrate, L-cysteine, thioacetamide and sodium molybdate. The invention relates to molybdyl modified I-III-VI2 group quaternary CuInZnS nano-crystal. The CuInZnS nano-crystal is a direct narrow bandgap and visible light responded quaternary semiconductor, the bandgap is continuous and adjustable, the quantum dot material has large specific surface area, more active sites are provided, the quantization advantage enables the semiconductor to obtain high charge migration rate, and the photocatalytic property is improved and enhanced; the molybdyl is electrostatically bonded with the sulfur vacancy on the surface of the nano-crystal and is modified on the CuInZnS nano-crystal, so the light absorbility of the nano-crystal is greatly improved, and the hydrogen producing activity is optimized.