Method for preparing polycarboxylic acid grafted copolymer superplasticizer

The invention discloses a method for preparing a polycarboxylic acid grafted copolymer superplasticizer. In the method, the preparation is implemented at the low temperature of 5-30 DEG C; a used initiating system is a complex initiating system which adopts the combination of one initiator I and various reducers R, or one reducer R and various initiators I or various initiators I and various reducers R, wherein the initiator (s) I is (are) selected from hydrogen peroxide, persulfate salt LS2O8 and a water-soluble azo initiator W, and the reducer (s) R is (are) selected from ascorbic acid and certain kinds of inorganic salt. The preparation method is simple, excellent in repetition stability and low in cost, does not require heating, and avoids toxicity and pollution; the prepared superplasticizer is high in water-reducing rate, and excellent in adaptability and slump-retaining capability, thereby being suitable for popularization and application.