The invention relates to a polycarboxylic cement grinding aid and a preparation method thereof. The polycarboxylic cement grinding aid comprises isopentenol polyoxyethylene ether, acrylic acid, methacrylic acid, 2-acrylamide-2-methylpropanesulfonic acid, sodium methylacrylsulfonate, sodium allysulfonate, hydrogen peroxide, ammonium persulfate, L-ascorbic acid, sodium formaldehyde sulfoxylate dihydrate, mercaptoacetic acid, mercaptopropionic acid, sodium hydroxide and water. The preparation method comprises the following steps: adding isopentenol polyoxyethylene ether, hydrogen peroxide, ammonium persulfate, sodium methylacrylsulfonate, sodium allysulfonate and water to a reaction kettle; processing acrylic acid, methacrylic acid, 2-acrylamide-2-methylpropanesulfonic acid and water to prepare a solution A, and processing L-ascorbic acid, sodium formaldehyde sulfoxylate dihydrate, mercaptoacetic acid, mercaptopropionic acid and water to obtain a solution B; and heating the reaction kettle to 30DEG C, adding the solution A and the solution B in a dropwise manner, and carrying out a polymerization reaction to obtain the final product. The molecular structure of the polycarboxylic cement grinding aid synthesized in the invention contains various functional groups, so the grinding efficiency is effectively improved, energy consumed in the grinding process is reduced, the early strength and the later strength of cement are improved, and the compatibility of cement and a concrete additive is guaranteed.