The invention discloses a copper-doped Fe3O4 magnetic composite nanomaterial as well as a preparation method and application thereof, and belongs to the field of magnetic nanoparticles and water treatment research. The copper-doped Fe3O4 magnetic composite nanomaterial (Cu-Fe3O4) is prepared by taking an iron source, a copper source and an alkali source as the raw materials to react in an ionic deep eutectic solvent by using an ionothermal method. The prepared Cu-Fe3O4 can be used as a multi-phase Fenton-like catalyst, has excellent capability of degrading organic matters (such as dyes) in the wastewater, can be easily separated and recycled magnetically, is stable in reusability (for example, the room-temperature degrading and decolorizing rate of the Cu-Fe3O4 on a rhodamine B solution serving as a frequently-used industrial dye still can reach up to 85% after the Cu-Fe3O4 is repeatedly used for 8 times, and k1 is 0.0132-1min). As a novel magnetic environment-friendly material, the copper-doped Fe3O4 magnetic composite nanomaterial is low in cost, quick and convenient to operate, obvious in effect and broad in application prospect.