The invention relates to the technical field of polycarboxylate water reducers for concrete and particularly relates to a hyperbranched polycarboxylate water reducer and a preparation method thereof. The preparation method comprises the following steps: mixing anhydride containing double bonds with amine compounds to react, so as to generate AB2 type monomers; reacting by virtue of the AB2 type monomers under the action of a water-carrying agent, so as to obtain a hyperbranched polyester intermediate; and mixing the hyperbranched polyester intermediate with a chain transfer agent, an initiator and an end-capping reagent to react, so as to generate the hyperbranched polycarboxylate water reducer. A synthetic process is simple, low in cost and beneficial to large-scale production.