The invention relates to a method for preparing organosilicon modified styrene/methyl methacrylate copolymer emulsion at room temperature which is characterized in that sodium dodecyl benzene sulfonate and p-octyl polyethylene phenyl ether are used as complex emulsifying agent and emulsion polymerization method is adopted at room temperature to prepare organosiloxane-styrene-methyl methacrylate high silica content copolymer emulsion. The optimal preparation technology is dertermined by using the particle size distribution of the copolymer emulsion and monomer conversion as indexes and adopting orthogonal experiment design method. The optimal preparation technology is as follows: the redox initiation system of cumene hydroperoxide and ferric sulphate, sodium hydrogen metabisulfite and disodium ethylenediaminetertraacetate is adopted, the common addition method of vinyltriethoxysilane which accounts for 21.3wt% of monomer is used, the total weight percent of the complex emulsifying agent is 2.00%, the stability of the polymerization is good, and the thermal stability of the terpolymer emulsion film is greatly increased compared with the pure styrene/methyl methacrylate two-componentcopolymer emulsion film.