The invention discloses an ester super sustained release polycarboxylic acid slump retaining agent and a preparation method thereof. The preparation method of the ester super sustained release polycarboxylic acid slump retaining agent comprises the following steps: (1) preparing methacrylic acid, saturated acid and 3-bis(2-hydroxyethyl)amino-2-hydroxypropanesulphonic acid under the action of a catalyst into unsaturated esterified small monomers; (2) stirring methoxypolyethylene glycol, methacrylic acid, concentrated sulphuric acid and beta-phenyl-naphthylamine in a flask, and reacting to obtain esterified monomers MP; and (3) stirring diethyl vinyl phosphate, a cationic photoinitiator A and water in a flask, mixing unsaturated carboxylic acid hydroxyalkyl ester monomers, the esterified monomers MP, unsaturated monocarboxylic acid, the unsaturated esterified small monomers and water, mixing a free radical polymerization photoinitiator B and water, mixing a chain transfer agent and water, irradiating the flask in ultraviolet light, and respectively adding the aqueous solutions into the flask dropwise, continuing reacting, and adding a sodium hydroxide aqueous solution for neutralizing to obtain the ester super sustained release polycarboxylic acid slump retaining agent. The slump retaining agent prepared by the preparation method has good slump retaining capacity for a long time.