The invention provides a preparation method of slump-retaining and shrinkage-reducing polycarboxylic acid. The method is characterized in that the slump-retaining and shrinkage-reducing polycarboxylic acid is prepared from a monomer A, a monomer B and a monomer C through free radical copolymerization, wherein the mol ratio of the monomer A, the monomer B and the monomer C is 1.0:(0.25-0.5):(0.25-0.5); the monomer A is an unsaturated monomer having carboxylic acid groups; the monomer B is an unsaturated polyether monomer of which the polymerizable active end is subjected to hydrophobic modification and the molecular weight is 300-1000, the monomer B has a hydrophobic-hydrophilic structure, and the HLB value of the monomer B satisfies that the HLB value is more than 10 and less than 20; and the monomer C is an unsaturated polyether monomer only containing an epoxyethane addition chain segment, and the molecular weight is 660-3200. The slump-retaining and shrinkage-reducing polycarboxylic acid provided by the invention is excellent in dispersion and dispersion retention properties and favorable in shrinkage reducing effect, can realize unified shrinkage reducing and dispersion properties in case of a low admixing amount, and can effectively improve the durability of concrete.