The invention relates to a preparation method of styrene-butadiene latex. The preparation method comprises main steps as follows: butadiene and styrene are taken as main monomers, modified monomers, a compound emulsifier, a redox initiator, an electrolyte, a dispersing agent and a molecular weight regulator are added, the components are subjected to two-stage polymerization including low-temperature polymerization at 5-15 DEG C and high-temperature polymerization at 50-70 DEG C, and when the conversion rate is higher than 90%, the reaction is stopped and ternary polymerization modified styrene-butadiene latex can be obtained. The preparation method of styrene-butadiene latex has main characteristics as follows: firstly, a straight-chain structure of low-temperature latex and high conversion rate of high-temperature latex are guaranteed simultaneously with adoption of high-low-temperature two-stage polymerization method, and secondly, a third monomer is added in a reaction process or after the reaction, so that a self-crosslinking structure of styrene-butadiene latex is formed. The styrene-butadiene latex is synthesized with a one-step method, the technology is simple, the control is easy, and performance of emulsified asphalt modified with the styrene-butadiene latex is remarkably improved.