The invention discloses a styrene-acrylic latex and its preparation method and application, belonging to the field of well cementing materials. The preparation method of this styrene-acrylic latex is, seed emulsion polymerization: styrene, acrylate, unsaturated acid or its salt, buffering agent, deionized water are added in the reactor, pass into nitrogen, add initiator, at 60 React at ℃-88℃ for 80-100min to obtain the seed emulsion polymerization part, and then separately add acrylates, and a mixture composed of initiator, buffer, unsaturated acid or its salts into the reactor at the same time, At the same time, the drops are completed, and the temperature is kept at 80°C-90°C to obtain styrene-acrylic latex. In the monomers of seed emulsion polymerization, styrene accounts for 14%-75%, and acrylates account for 24%-85%; in the monomers of soap-free emulsion polymerization, acrylates account for 92%-99%; seed emulsion The percentage of the polymerized portion to the total polymer is 5%-25%. The preparation method has simple process and short time consumption, and the prepared styrene-acrylic latex has good stability and can greatly improve various properties of cement slurry.