The invention discloses a method for synthesizing MBS resin. The method for synthesizing an impact transparent modifier, namely a butadiene, styrene and methyl methacrylate terpolymer (MMA) (short forMBS resin), for PVC resin by using a reaction type emulsifier sodium p-styrenesulfonate comprises the following steps: synthesizing styrene-butadiene latex by taking the sodium p-styrenesulfonate asan emulsifier, wherein butadiene accounts for 60 to 90 percent by mass of the total amount of monomers; and grafting styrene and methyl methacrylate to the styrene-butadiene latex in two steps, in thefirst step, adding the styrene with all formula quantity and the methyl methacrylate with 1/3 of amount, and in the second step, adding the remained methyl methacrylate. The comprehensive physical property of the MBS resin prepared by the process is equal to that in the existing process; meanwhile, the polymerization system is obviously improved, the gum productivity is reduced and the yield of the product is increased.