The orthogonal experiment method was introduced into the formulation optimization of polymer lotion waterproof coating in order to reduce the number of experiments and obtain the best formulation. The water absorption, tensile strength and elongation at break of the waterproof coating were taken as the investigation points of the experiment, and the orthogonal test was carried out with the brand of lotion, the proportion of lotion and filler, and the mass fraction of sericite powder as the main factor variables, so as to obtain a coating formula with better performance. The results showed that the coating formulation optimized by the orthogonal test method met the standard of JC/T 864—2008 Polymer Lotion Building Waterproof Coating. The water absorption was 14.76%, the tensile strength was 1.63 MPa, and the elongation at break was 336.57%.