The invention discloses a preparation method for a modified graphene-loaded titanium dioxide composite photocatalyst. The method comprises the following steps: obtaining a graphene oxide suspension through an improved Hummers method, and adding triethanolamine into the graphene oxide suspension for reacting to obtain a modified graphene oxide suspension liquid; dropwise adding the obtained modified graphene oxide suspension liquid into a sulfuric acid solution in which titanyl sulfate is dissolved; performing ultrasonic auxiliary soaking, and then performing centrifugal separation to obtain a composite photocatalyst precursor; adding the composite photocatalyst precursor into an alkali solution to perform hydrothermal reaction; pre-drying a hydrothermal reaction product, and performing high-temperature calcining and reducing in an air atmosphere to obtain the modified graphene-loaded titanium dioxide composite photocatalyst which is stable in structure, high in specific surface area, high in photocatalytic activity, small in titanium dioxide particle size (about 10 nm), uniform in distribution, and difficult to agglomerate. The method is easy to operate and low in cost, and meets industrial production.