The invention provides a preparation method of a hyperbranched polycarboxylic acid copolymer cement dispersant. A monomer A, a monomer B and a monomer C are subjected to radical copolymerization reaction in an aqueous medium, and the molar ratio relations among the monomer A, the monomer B and the monomer C meets the following conditions: B/A=2-10, and C/(A+B+C)=0.02-0.08, wherein the monomer A is represented by a general formula (1), the monomer B is represented by a general formula (2), and the monomer C is represented by a general formula (3). The monomer C is a sulfhydryl-containing macromolecular monomer. When the monomer C participates in polymerization, sulfhydryl groups in the molecular structure of the monomer C perform a chain transfer function on polymerization, and the chain transfer function leads the sulfhydryl-containing side chains of one comb polymer chain to be connected with the bottom end of the main chain of another comb polymer chain, thus, a hyperbranched structure is formed by crosslinking multiple times. The hyperbranched polycarboxylic acid copolymer cement dispersant prepared with the method has excellent capability of dispersing cement, higher water reducibility and excellent slump keeping capability under the condition of smaller doping quantity.