This invention relates to a synthesis method for carboxylated styrene-butadiene latex used in the cement-based waterproof coating. The preparation method involves 30 to 50 shares of butadiene or isoprene isoprene, 30 to 55 shares of styrene, 0.1 to 5 shares of methyl acrylate , 1 to 5 shares of itaconic acid, 1 to 8 shares of n-butyl acrylate butyl acrylate, 1.2 to 2.8 hares of emulsifier, 0.3 to 1.2 hares of initiator potassium supersulphate, 0.2 to 0.8 shares of molecular weight regulator, 0.01 to 0.5 shares of pH buffer sodium bicarbonate, 0.01 to 5 shares of electrolyte, 0.01 or 5 shares of chelating agent ethylenediamine tetraacetic acid or endrate disodium and 100 to 150 shares of soft water for polymerization. The electrolyte is the combination of potassium phosphate and disodium hydrogen phosphate, with the ratio of 3 to 2: 1; in the polymerization reaction, monomer and a variety of agents are added with intermission. And reaction temperature control adopts the two-step laddered control. When the conversion rate reaches 99.0 percent, the mixture experiences the vacuum degassing treatment. The latex produced by the present invention is of good compatibility with the additives as the paint during the paint preparation. The prepared paint is of excellent fluidity, water retention and machinery stability under the high-cut condition. The improved mortar has the advantages of good permeability, high bond strength and so on.