The effects of formate salts on the rheological characteristics of water-based solutions were compared for two laboratory research methods, i.e., dynamic theological analysis at high temperature and high pressure (HTHP) and a static hot-roll aging test. Polymer muds were used for the measurements. HTHP rheology analysis showed that sodium formate retained effectively the dynamic viscosity at HTHP. Hot-roll tests showed that sodium formate had an obvious effect on the rheological characteristics of the polymer fluid at mom temperature after preliminary heating. The filtration loss of the polymer fluid with added sodium formate was slightly greater than that of pure polymer fluid although the filtration loss could be controlled by changing the formate concentration. Test results from both methods showed that sodium formate could be used successfully as a polymer drilling fluid stabilizer under HTHP operating conditions.