The invention relates to the technical field of compound preparation, and provides a method for preparing mandelic acid from trichloroisocyanuric acid chlorinated styrene. The invention uses styrene as a basic raw material, trichloroisocyanuric acid as a chlorination agent, and ethylene glycol A mixed solution of alcohol dimethyl ether, methanol, ethanol, and water is used as a solvent, and the strong oxidizing and chlorinating properties of trichloroisocyanuric acid are used to directly oxidize and chlorinate styrene into 2,2-dichloroacetophenone in one step , and then hydrolyze 2,2-dichloroacetophenone to obtain mandelic acid. After the reaction of trichloroisocyanuric acid, cyanuric acid is generated to be recovered. Compared with the prior art, the present invention has significant advantages: (1) adopts cheap, less toxic and non-irritating styrene as raw material. (2) Trichloroisocyanuric acid reacts to generate cyanuric acid, which can be chlorinated to generate trichloroisocyanuric acid for recycling. (3) The production process is short, the waste discharge is less, and it is environmentally friendly and green.