The invention relates to a preparation method of styrene-butadiene latex used for modification of emulsified bitumen. According to the preparation method, butadiene and styrene are taken as main monomers, a compound emulsifier, a redox initiator, an electrolyte, a dispersing agent, and a molecular weight modifier are added; low temperature (5 to 15 DEG C) polymerization is carried out, wherein the monomers are added in batches at intervals; and when the conversion rate is larger than 90%, reaction is carried out for 12 to 16h, and then is stopped so as to obtain styrene-butadiene latex with a solid content higher than 50%. According to the preparation method, the structure of latex particles is adjusted so as to improve the compatibility of latex with bitumen, and latex structure gel content is controlled so as to prepare styrene-butadiene latex used for modification of emulsified bitumen for different performance requirements; one step synthesis is adopted; the technology is simple, and is convenient to control; and the product performance is excellent.