The invention discloses a method for synthesizing polycarboxylate superplasticizer through a RAFT method. According to the method, firstly, macro-monomers A are added into a reaction kettle, water is added, water bath heating is performed till the temperature is 65-85 DEG C, and constant-temperature stirring is performed; then, a trithio ester chain transferring agent is dissolved in a mixed solution of micro-monomers B and water, the mixture is dropwise added into the reaction kettle at constant speed, and meanwhile an initiator aqueous solution is dropwise added; after all the needed matter is dropwise added, a stirring reaction lasts for 3-5 h under the continuous constant-temperature stirring condition of 65-85 DEG C; after the reaction ends, an alkali solution and water are added to adjust PH to 6-7, and the polycarboxylate superplasticizer is obtained after reaction products are cooled to room temperature. By means of the method, the molecular-structure-controllable polycarboxylate superplasticizer can be prepared, the molecular weight is controllable, the molecular weight distribution range is narrow, polymer molecule design can also be performed, and the method has great significance in research and development of the polycarboxylate superplasticizer synthetic technology.