The invention relates to a polymerization method capable of increasing an ABS grafted copolymer conversion ratio. The method includes: adding an emulsifying agent, an electrolyte and an aqueous solution of an initiator according to a ratio into a polymerization kettle, vacuumizing, adding a butadiene monomer to obtain polybutadiene latex, adding the polybutadiene latex, an emulsifying agent, an initiator and water into a polymerization kettle, adding an emulsified liquid of styrene, acrylonitrile and lauryl mereaptan in a continuous feeding manner, adding a bifunctional active monomer after dropwise addition is finished, adding an aqueous initiator and an antioxidant, adding into an aqueous sulfuric acid solution having a concentration of 0.1-5% for condensation, performing condensation, drying to obtain ABS grafted powder, mixing the ABS grafted powder with SAN according to a ratio, extruding by an extruder, and granulating to obtain a finished product of ABS resin. The method increases the conversion ratio of grafted polymerization to more than 98%, increases the utilization efficiency of devices, reduces pollution to the environment, and reduces the production cost.