Provided is a method of preparing a powdery diacetal clarifying agent, which comprises mixing an aromatic aldehyde, a polyol, and an acid catalyst in an organic polar solvent, adding a hydrogenating agent and an inorganic silicon-containing agent into the foregoing mixture, and filtering the mixture. The powdery diacetal clarifying agent prepared by the method can have excellent flowability, dispersability, thermal resistance, and color stability. Accordingly, the powdery diacetal clarifying agent does not release stinking odor and incur yellowing at high temperature, allowing the plastic articles to have improved appearance and visual appeal.