Abstract: At high temperature and high pressure (HTHP), the density of water-base drilling fluid is not a constant any more. Using the HTHP static density measurement device, the variation of the static density of different density water-base drilling fluid is studied, i.e., p-T-V or the p-T-ρ is studied, which is determined by the pressure and the temperature. The mathematic model indicating the variation of the static density along with the pressure and the temperature is set up. The factors affecting the variation of the density of water-base drilling fluid at HTHP are analyzed. It is concluded that the temperature exerts the greatest effect on the density of water-base drilling fluid, while the pressure exerts the least. Furthermore, it is considered that the influence of the pressure on the density gets greater as the temperature increases. The actual density of the drilling fluid at HTHP is exactly predicted, which is favorable for the accurate prediction and control of the downhole pressure. So the safe drilling of oil and gas wells is guaranteed.