The invention discloses an ester slump retaining type polycarboxylate water reducer and a preparation method thereof. The ester slump retaining type polycarboxylate water reducer is made from raw materials comprising the following components in parts by weight: 100 parts of esterified macromer solution by weight, 2-6 parts of unsaturated monomer by weight, 1.5-4 parts of unsaturated phosphate by weight, 1-4 parts of an oxidizing agent by weight, 0.9-3.5 parts of a reducing agent by weight, 1-3 parts of a chain transfer agent by weight, 108 parts of water by weight, and 20 parts of industrial sodium hydroxide solution by weight, wherein the concentration of the esterified macromer solution is 75-85%, and a solute thereof is the mixture of methoxy polyethylene glycol methacrylate, crosslinking monomers triethylene glycol dmethylacrglate and methyl methacrylate; the unsaturated monomer is methacrylic acid, acrylic acid, hydroxyethyl methylacrylate, hydroxyethyl acrylate, hydroxypropyl methacrylate or hydroxypropyl acrylate. The ester slump retaining type polycarboxylate water reducer provided by the invention has good slump retaining performance, good dispersibility, good adaptability and low cost.