A microemulsion viscosifier AM/AA/AMPS/DMAM was synthesized by inverse microemulsion polymerization with acrylamide (AM), acrylic acid (AA), 2-acrylamido-2-methylpropane sulfonic acid (AMPS) and N, N-dimethylacrylamide (DMAM) as monomers. The microemulsion could be added into the drilling fluids directly. The apparent viscosity(AV), plastic viscosity(PV), yield point(YP) and fluid loss(FL) of drilling fluids were 26.5 mPa·s, 16.0 mPa·s, 10.5 Pa and 14.2 mL accordingly when the addition amount of microemulsion was 0.6% at room temperature. The AV, PV and YP of drilling fluids showed downtrend and FL increased with increasing rolling temperature when the addition amount of microemulsion was 1.0%. But the variation was minor with the temperature less than 150℃, which showed good temperature tolerance. The NaCl resisting concentration of the drilling fluids could reach saturation and CaCl2 resisting concentration was 0.6% when 2.0% microemulsion added. Compared with the conventional viscosifier 80A-51, the temperature and salt tolerance of the synthesized polymer had been improved obviously.