The invention discloses a method for preparing a modified hydroxyl ester polymer emulsion. Required raw materials comprise the following components in parts by mass: 1.0-4.2 parts of a hydrophilic small monomer for emulsification, 1.7-4.5 parts of a hydrophobic small monomer for emulsification, 18-51 parts of a hydroxyl small monomer, 0.05-0.26 part of an emulsion initiator, 2.2-5.4 parts of an initiator a, 3.5-6.6 parts of an initiator b, 380-450 parts of an ester small monomer, 6.2-9.3 parts of a crosslinking agent, 0.06-0.28 part of a chain transfer agent, 0.5-1.0 part of graphene oxide, 15-40 parts of polyether, 5-10 parts of a neutralizer, 5-10 parts of a pH regulator, 2.0-6.5 parts of an emulsifier promoter, 0.4-1.3 parts of a liquid film enhancer and the balance of water, wherein the total parts by mass are 1000, and the solid content is 50%. The waterproof coating disclosed by the invention is suitable for cement mixing; due to a large number of hydroxyl groups, calcium ions can be stably complexed, calcium ion hydrolysis can be effectively inhibited, the cement strength can be increased, and the waterproof effect can be improved; due to hydrogen bonds in molecules of the hydroxyl groups, the internal binding force of the polymer can be improved, and the adhesion of the polymer with a matrix can be improved through a good hydrophilic effect; due to addition of the graphene oxide, the tensile strength of a polymer emulsion in film formation can be remarkably improved; and the water resistance can be also improved to a certain extent.