The invention relates to a preparation method of a comb copolymer shrinkage reducing agent. The comb copolymer shrinkage reducing agent is prepared by carrying out radical copolymerization and polymerization of a monomer A: ether macromonomer containing unsaturated double bonds, a monomer B: (methyl) crylic acid or salts thereof, a monomer C: alkoxyl polyether (methyl) acrylic ester and a monomer D: maleic anhydride monoester or diester under the action of an initiator, and regulating the pH value to 6.0-8.0 after the reaction is ended, wherein the mole ratio of the monomer A to the monomer B to the monomer C and the monomer D is 1.0: (0.2-1.0): (0-1.0): (0-1.0), the monomer A and the monomer B are necessary, and the monomer C or the monomer D can both be zero, but not necessary simultaneously. The preparation method has simple synthesis process, is easy to control reaction process and can not generate crosslinking phenomenon, and the like. The invention achieves better shrinkage reduction effect under the lower doping amount, overcomes the defect of strength reduction of an alcohol shrinkage reduction agent and a polyethers shrinkage reduction agent and has the compression strength at 28d larger than the strength of standard concrete.