The invention relates to a preparation method of a g-C3N4 quantum dot modified iron oxide photocatalyst, which is characterized by comprising the following steps: 1, mixing ferric chloride and 1,2-propane diamine, performing a reaction in a reaction kettle at 200-230 DEG C for 2-3 hours, washing, vibrating and centrifuging the product, and washing and drying the precipitate to obtain an iron oxidenanosheet; 2, putting melamine and urea into a crucible, preserving heat for 1.5 to 3 h at 500 to 530 DEG C in an N2 atmosphere to obtain g-C3N4, grinding the g-C3N4 nanosheet, adding the g-C3N4 nanosheet into a mixed solution of concentrated sulfuric acid and concentrated nitric acid, performing a reaction at 40-60 DEG C for 2-3 hours, cooling, centrifuging and drying the reaction product to obtain the g-C3N4 nanosheet, dispersing the g-C3N4 nanosheet into a mixed solution of deionized water and ethylene glycol, performing ultrasonic treatment at 60-80 DEG C for 20-24 hours, performing standing treatment, centrifuging the mixture, and washing and drying the solid product to obtain the g-C3N4 quantum dot; 3, dispersing the iron oxide nanosheets into deionized water, uniformly stirring theliquid, adding CNQDs, transferring the mixture into a reaction kettle, performing a reaction at 150-200 DEG C for 2-5 hours, and cooling, washing and drying a finish product. The preparation method has the beneficial effects that the photocatalysis efficiency is improved, and the defect that the quantum yield of a single iron oxide nano material is relatively low is overcome; the preparation method is simple and easy to implement, high in repeatability and low in cost.