The invention belongs to the field of material technology, relates to a fluorescent powder used for LED, and in particular to a monophase fluorescent powder emitting white light under the excitation of near ultraviolet rays and a preparation method thereof. The preparation method comprises the following steps of: preparing a gel precursor of dysprosium-doped dodecacalcium heptaluminate by using achemical coprecipitation method, dissolving the raw materials of high-purity Dy2O3, Al(NO3)3.9H2O and Ca(NO3)2.4H2O in deionized water in a proportion, dropwise adding a right amount of nitric acid, heating and uniformly stirring; adding a right amount of NH3.H2O as a precipitant, drying the obtained gel precursor to remove water, and sintering to obtain the dysprosium-doped dodecacalcium heptaluminate fluorescent powder. When dodecacalcium heptaluminate is used as a rare earth dysprosium-doped matrix material, because the band gap width of dodecacalcium heptaluminate can reach 5.9 eV, the temperature quenching effect of dysprosium light emission is weakened, which is beneficial to the enhancement of white light emission of dysprosium. Proved by results, the dysprosium-doped dodecacalciumheptaluminate powder is expected to be applied to the fields of display devices, lighting devices, photoelectric devices and the like.