The invention belongs to the technical field of concrete additives and in particular relates to preparation of macromonomer-diallyl polyethylene glycol (DAPEG) and a novel polycarboxylic acid water reducing agent prepared by using DAPEG as a raw material. The water reducing agent is prepared by carrying out heating reaction on the DAPEG, maleic anhydride (MA), sodium allyl sulfonate (SAS) or methyl sodium allyl sulfonate (MSAS), ammonium persulfate (APS), sodium hydroxide and water according to proper mass proportion by using nitrogen as shielding gas. In the invention, the polycarboxylic acid serial water reducing agent is prepared from the DAPEG for the first time, thus the variety of the polycarboxylic acid serial water reducing agent is enriched; and meanwhile, the prepared polycarboxylic acid serial water reducing agent has favorable performance, such as low doping amount, high water reducing rate, no delayed coagulation, low slump loss, and the like, thereby having more favorable market prospect and economic benefits.