The invention relates to a synthesis method of drilling fluid filtrate reducer. The method comprises: a, nonionic compound emulsifying agent is dissolved in oil and oil phase is made up; a first monomer chosen from fumaric acid, maleic acid, itaconic acid or crylic acid, a second monomer chosen from N, N- dimethylacrylamide, N, N-diethylacrylamide or acrylamide and a third monomer chosen from 4-vinylbenzenesulfonic acid sodium salt, sodium allylsulfonate, 3-allyloxy-2-dydroxy-1-sodium allylsulfonate, 2- methacryloxypropyl-2-methyl propanesulfonic acid, 2-acrylamido-2-methyl propanesulfonic acid or SMAS are dissolved in water with the molar ratio of 1:1 to 5:1 to 5, the pH value of the solution is adjusted to be between 8 and 11, evocating agent is added and water phase solution is made up; and b, the water solution is added to the oil phase solution and the temperature rises to 40 to 70 DEG C for polymerization. The method of the invention has the advantages of fast polymerization speed, high outcome molecular weight and high outcome stability, good filtrate reducing performance, high temperature resistance and high salt resistant performance.