The invention provides a polycarboxylic acid concrete plasticizer which is formed by copolymerization of three monomers A, B and C and is a polymer with the weight-average molecular weight of 15000-40000, wherein the three monomers are as follows by weight percent: monomer A is 60-70%, monomer B is 20-30% and monomer C is 5-10%; the structure of the monomer A is shown by the following formula, wherein R1 is alkyl group with less than or equal to five carbon atoms or hydrogen atom; m and n are integers within the range of 0-5; A, B, C, D , E, G, H, J and K are respectively integers within the range of 0-30, but not simultaneously 0; the monomer B is the mixture of more than two of acrylic acid, methacrylic acid, maleic acid, maleic anhydride or maleic anhydride salt; and the monomer C is the mixture of more than two of sodium methyl acryl sulfonate, sodium vinylsulphonate, styrene sodium sulfonate or 2-acrylic amide-2-methyl acrylic acid. The concrete plasticizer has the characteristics of good compatibility with various cements, and high water reducing rate and anti-collapse performance. The invention also provides a preparation method of the plasticizer.