The invention discloses a chromium doped alumina sheet shape self-assembly microballoon luminescent material and its preparation method.The microballoon is formed by a sheet shape structure with adjustable thickness through self-assembly, the particle size of the microballoon is uniform, the monodispersion is good, the diameter of the microballoon is 0.8-1.0 mum, the thickness of a sheet shape self assembly structure unit is 10-20 nm, the invention is characterized in that the fiber material comprises alumina and chrome oxide according to mol ratio of 0.01: 99.99-0.1: 99.9. When the excitation wavelength is 462 nm, the characteristic radiation is generated while the chromium doped alumina sheet shape self-assembly microballoon is at 696 nm. The thickness of the particle size of the products and sheet shape self-assembly unit can be effectively regulated and controlled by adjusting the types and proportion of the solvent. The luminescent property of the material of the invention enables a large application value on the aspects of fluorescent material and the like. The method of the invention has the advantages of simple process, safe operation and low cost.