The invention provides a method for preparing a high-entropy oxide material (MAlFeCuMg)3O4 by a hydrothermal method, which comprises the following steps of: respectively dissolving cobalt nitrate / chromic nitrate, aluminum nitrate, copper nitrate, ferric nitrate and magnesium nitrate in ultrapure water according to equal moles, and fully and uniformly stirring to obtain a mixed solution of metal salt; adding a precipitant into the mixed solution of metal salts, uniformly stirring, transferring into a stainless steel high-pressure reaction kettle with a polytetrafluoroethylene lining, carrying out a hydrothermal reaction, cooling to room temperature after the reaction is finished, carrying out reduced pressure filtration, ultrapure water washing, suction filtration, separation, precipitation and drying on the reaction liquid, and crystallizing the obtained solid powder to obtain the high-entropy oxide (MAlFeCuMg)3O4, wherein M is Co or Cr. The high-entropy oxide is of a spinel structure, has high infrared emissivity at the wave band of 3-12 microns, and can be suitable for infrared heating. The metal salt is used as a raw material and is wide in source and low in cost; a hydrothermal method is adopted, it can be ensured that several metal elements are fully and evenly mixed, and the reaction conditions are mild and environmentally friendly.