The invention belongs to the field of a preparation method for heterocyclic compounds, and in particular relates to a method for synthesizing 5,5-diphenyl-2-thiohydantoin by taking benzil and thiourea as original raw materials under an alkaline condition. The method comprises the following steps of: (1) adding the benzil, the thiourea, alkali and a solvent into a reaction kettle; (2) performing areflux reaction under a normal pressure condition; and (3) after the reaction ends, cooling, diluting, acidifying, separating and recrystallizing reaction liquid to obtain a target product, wherein in the step (1), the alkali is one or the mixture of hydroxide and carbonate; and the solvent is one or the mixture of more than two of water, dimethyl sulfoxide, acetone, tetrahydrofuran, acetonitrile, dioxane, methanol, ethanol and isopropanol. The invention has the advantages of readily available raw materials, temperate reaction condition, high yield, low cost and suitability for industrial mass production.