The invention relates to the field of polymers, and in particular relates to modified carboxylated styrene-butadiene latex and a preparation method thereof. The preparation method of the modified carboxylated butyl styrene latex includes the following steps: (1) in the presence of an initiator, performing polymerization reaction on a mixed system of butadiene, styrene, acrylic acid, a retarder, amolecular weight modifier, water, a pH buffer, a chelating agent and an emulsifier sequentially at 55-60DEG C and 65-75DEG C; (2) mixing a polymerizing reaction product with an oil phase containing anitrile-based functional monomer, styrene and butadiene, and an aqueous phase containing water, an initiator, an emulsifier, and a molecular weight modifier, and performing polymerization reaction onthe resulting mixture sequentially at 75 DEG C to 80DEG C and 85 DEG C to 90 DEG C. The modified carboxylated styrene-butadiene latex prepared by the method provided by the present invention not onlyhas a small particle size, but also has excellent stability and extremely high adhesive strength.