In this paper, phosphoric acid and propylene oxide were used as raw materials, and a reactive flame retardant tris(2-hydroxypropyl) phosphate was synthesized by using a Lewis acid ionic liquid as a catalyst in a green solvent. The structure was characterized by infrared, nuclear magnetic and mass spectrometry. The effects of the type and amount of catalyst, material ratio, reaction time and reaction temperature on the reaction were studied. The results show that the optimum conditions are n (phosphoric acid): n (propylene oxide) = 1:3.2, the reaction temperature is 90 ? C, the reaction time is 3 h, the amount of the catalyst is 3%, and the yield of the obtained product can reach 92.8%. The product performance index is excellent, and the thermogravimetric analysis shows that its stability meets the material processing requirements.