The invention relates to a synthetic method for polyoxyethylene castor oil, and belongs to the synthetic technical field of an organic compound. Castor oil and ethylene oxide are used as the raw material, and the polyoxyethylene castor oil is produced through polyreaction with the existence of a catalyst; the weight ratio between the castor oil and the ethylene oxide is 1: 0.145 to 2.98; the catalyst is any one component or the mixture of more than one component in solid sodium methoxide, sodium methoxide methanol solution, KOH, and NaOH, etc., and the adding amount of the catalyst is 0.05 to 0.2 percent of the weight percentage of the produced polyoxyethylene castor oil. Technological parameters and conditions such as the raw material proportioning, the adding amount of the catalyst, the polyreaction temperature and time, etc. are reasonably determined, the reaction condition is temperate, and the requirement on the equipment is low. The yield rate is high, no impurity can be generated, and the invention is environment friendly. The polyoxyethylene castor oil produced by the invention has light color, transparent appearance, good level dyeing performance, strong compatibility and wide molecular weight adjustment range.