The invention discloses a preparation method of a carbon quantum dot modified n-n type MFe12O19 / CeO2 magnetic separation photocatalyst, wherein the preparation method comprises the steps: by using glucose as a raw material, synthesizing carbon quantum dots in a reaction kettle; using M metal salt, ferric salt or cerium salt as a raw material, adding a chelating agent, an anti-gel collapse agent, acrylamide and a cross-linking agent step by step, and polymerizing acrylamide and the cross-linking agent under the action of a field, to form gel; carrying out drying, sintering and other processes on the gel to synthesize MFe12O19 or CeO2 nanoparticles; ultrasonically stirring a proper amount of carbon quantum dots, MFe12O19 and CeO2 nanoparticles in an alcohol solution, pouring out supernatantliquid, drying, transferring into a box-type furnace, sintering, and adjusting the ratio of the carbon quantum dots, MFe12O19 and CeO2 nanoparticles, to obtain the n-n type MFe12O19 / CeO2 magnetic separation photocatalyst modified by the carbon quantum dots and having controllable photocatalytic activity. The photocatalyst is low in cost and easy to synthesize, and overcomes the defects that a traditional photocatalyst is low in charge separation efficiency and difficult to recycle.