The invention relates to a preparation method of styrene-butadiene latex for modifying emulsified asphalt, comprising the main steps of using butadiene and styrene as main monomers, adding a composite emulsifier, an oxidative reduction initiator, electrolyte, a dispersant, and a molecular weight regulator, and synthesizing styrene-butadiene latex with a solid content of 55% and above by using ultrasonic-assisted seeded emulsion polymerization technique. The preparation method has the main advantages that ultrasonic equipment is used for assisting, the usage of the initiator is reduced, reaction temperature is lowered, and the molecular weight of latex is increased; the technique for the prepared styrene-butadiene latex is simple, and the styrene-butadiene latex is easy to control, is used mainly for modifying emulsified asphalt, and has significant modifying effect.