The invention relates to the technical field of polymers, specifically to a waterborne anti-sagging polymer and a preparation method thereof. The waterborne anti-sagging polymer provided by the invention is prepared from the following main raw materials by mass: 35 to 50% of a cosolvent, 0.4 to 1.5% of mercaptoethanol, 1.8 to 3.5% of a hydrophilic monomer, 20 to 33.5% of a vinyl monomer, 0.05 to 0.6% of an initiator, 1.8 to 4.2% of diisocyanate, 0.35 to 0.7% of a polyamine compound, 2 to 4.3% of a neutralizing agent and 16.3 to 27% of deionized water. The waterborne anti-sagging polymer prepared by using the preparation method provided by the invention contains a polyurea group which can generate a hydrogen bond with main resin in coating components to form a reversible three-dimensional structure, so the viscosity of a system is improved, and an anti-sagging effect is achieved.