The invention relates to method for synthesizing castor oil polyoxyethylene polyoxypropylene ether, and belongs to the technical field of synthesis of organic compounds. Raw materials comprise castor oil, ethylene oxide and epoxypropane, A Mg-Al catalyst is added to the castor oil, and vacuum dehydration is carried out for 1-3 hours; ethylene oxide is added, temperature is kept at 110 DEG C to 180 DEG C, epoxypropane is added after curing is carried out for 1-5 hours, and curing is carried out for 3-10 hours at the temperature of 110 DEG C to 180 DEG C; the temperature is reduced to 80 DEG C to 120 DEG C, after vacuum degassing is carried out for 0.5-3 hours, the temperature is reduced to 40 DEG C to 80 DEG C, and discharging is carried out to obtain finished castor oil polyoxyethylene polyoxypropylene ether. The method is applied to synthesizing castor oil polyoxyethylene polyoxypropylene ether, and synthesized castor oil polyoxyethylene polyoxypropylene ether has the advantages of being light in color and luster, transparent in appearance, excellent in physicochemical index, high in compatibility performance, large in molecular weight control range and the like, can be used for the industries of daily use chemicals, medicine and the like with high requirements.