Abstract：Redox iniator was used to conduct the copolymerization of AM, DMDAAC and AMPS. The terpolymer was then hydrolyzed with NaOH under 85 ℃ to synthesize hydrolyzed polymer. The structure of terpolymer and hydrolysate was characterized by IR and NMR, while the thermal stability of those products was studied using TGA. The increment of hydrolysis degree not only improved the thermal stability of hydrolysate but also enhanced the apparent viscosity, plastic viscosity and yield point of drilling fluids. An optimum content of hydrolysis degree, however, was found to reduce both API and HTHP filtration to minimum values. Mud was prepared by adding hydrolyzed polymer with such an optimal proportion of COO- of 3.5 wt%, bentonite of 5 wt% and NaCl of 30 wt%. API filtration and HTHP filtration was 14.0 mL and 40.0 mL respectively, after the mud was rolled for 16 h under 200 ℃.