The invention discloses a method for producing THEIC. The method comprises the following steps: enabling cyanuric acid and ethylene oxide to be subjected to addition reaction in an organic solvent under the catalysis reaction of a catalyst, decoloring, filtering, cooling and crystallizing the reaction solution after the reaction is completed, so as to obtain a target product, wherein the organic solvent is methanol or ethanol, the catalyst is a solid-liquid mixed catalyst containing a solid catalyst and a liquid catalyst, the solid catalyst is tetramethylammonium chloride, and the liquid catalyst is triethylamine. According to the technical scheme of the invention, the comprehensive yield of the product can be increased from 88% of the prior art to 95% of the prior art, the product quality is greatly improved, the production period is shortened, the product yield is increased, the use frequency of the solvent are increased to be more than 80 times from the conventional more than 30 times, the raw material consumption is effectively reduced, the reaction time is shortened, the product quality can be further ensured, and guarantee for producing THEIC with high quality is provided.