The invention discloses a method for separating and assaying impurities in apremilast and preparations thereof through liquid chromatography. In the method, octadecylsilane chemically bonded silica is employed as a filler in a chromatography column; a buffer solution is employed as a mobile phase A; and a methanol-acetonitrile mixture solvent is employed as a mobile phase B, wherein a gradient elution method is employed in the mobile phases to assay the impurities in apremilast and the preparations thereof. The method can effectively separate and assay unknown impurities and known impurities from the apremilast. The method has strong specificity, is high in accuracy, is easy to use and can be used for effectively control the quality of the apremilast and preparations thereof.