The invention belongs to the field of preparation of composite catalytic materials, in particular relates to a graphene / copper hydroxyphosphate composite catalyst and a preparation method thereof. The specific steps are as follows: 1) preparing a graphene oxide water solution with certain solubility; 2) adding a certain amount of graphene oxide water solution into a soluble copper salt and soluble phosphate reaction system; 3) adjusting the pH value; 4) putting the suspension into a reactor for reaction for 6-12h at 100-140 DEG C, cooling, centrifuging, washing and drying to obtain the graphene / copper hydroxyphosphate composite catalyst. The mass ratio of graphene nano sheet to copper hydroxyphosphate in the graphene / copper hydroxyphosphate composite catalyst is 0.2-6%: 99.8-94%, and optimally is 2%: 98%. The composite catalyst has good catalytic performance under simulated sunlight, and can be applied to the field of water treatment of industrial wastewater.