The invention discloses a preparation method of an ester type polycarboxylate water reducer. The method comprises the following steps: 1, adding polyethylene glycol monomethyl ether methacrylate, acrylic acid, unsaturated sulfonate, paraffin, ethylenediamine tetraacetic acid disodium salt and water into a reaction kettle, stirring for dissolving to prepare a reaction bed material, continuously introducing nitrogen under the liquid level of the reaction bed material, and heating to 30-40DEG C; 2, adding hydrogen peroxide to a material obtained in step 1 in a one-time manner, simultaneously adding a first mixed solution, a second mixed solution and a third mixed solution in a dropwise manner under a controlled dropwise addition speed for 1-2.5h, heating the reaction system to 85-95DEG C after the dropwise addition ends, adding ammonium persulfate in a one-time manner, and continuously reacting; and 3, adding sodium dimethyldithiocarbamate to a material obtained in step 2 to end the reaction, adding a sodium hydroxide solution to neutralize, cooling to 40-50DEG C, and discharging to obtain the ester type polycarboxylate water reducer. The preparation method is in favor of avoiding gel generation, and greatly improves the storage stability of the prepared polycarboxylate water reducer.