The invention relates to a method for improving the stability of fluorescent powder. The method comprises the following steps: taking the fluorescent powder or the fluorescent powder subjected to surface treatment; adding the fluorescent powder into an organic solvent containing polysiloxane, tetraethyl orthosilicate or derivative thereof and dibutyltin dilaurate, stirring and hydrolyzing until the organic solvent is volatilized completely to obtain a mixture; adding azodiisobutyronitrile into the mixture, refluxing and heating, and performing polymerization reaction to obtain stable fluorescent powder subjected to hydrophobic modification. The method is simple in flow, and the hydrophobic fluorescent powder with high stability can be prepared.