Drilling fluid density is very sensitive to the variation of temperature and pressure, especially in deep water and high temperature/high pressure environment. As downhole environment is different from the environment at surface, there must be a difference of density under these two conditions. The accuracy of downhole density calculation is directly related to whether the true downhole hydrostatic pressure could be reflected or not. Drilling fluid could be compressed under high pressure as well as be expanded under high temperature conditions. This paper proposes two important relational equations based on isothermal compressibility and isobaric expansivity. One equation is the relationship between isothermal compressibility and pressure. In this case fluid's volumetric reduction under certain pressure increment could be obtained. The other equation is the relationship between isobaric expansivity and temperature. Similarly, fluid's volumetric increment with the increasing temperature could be gotten by this equation. Therefore the true hydrostatic pressure at downhole could be accurately described by solving these two equations. Experiment data results show that curves of isothermal compressibility with pressure and isobaric expansivity with temperature are both conformed to exponential form. Besides, there is a special set of temperature and pressure for a given drilling fluid. Under this special situation, the compressibility are exactly offset by thermal expansivity, then mud density remain unchanged. Field applications indicates that this density model can describe downhole hydrostatic pressure well at Northwest China and South China Sea. Through adjusting mud density by this model, the headache problem of shallow narrow mud density window formation at South China Sea is well overcome. In addition, the true downhole equivalent static density (ESD) is smaller than fluid density measured at surface in a HP/HT well of Northwest China, whereas the situation is just the opposite in a deepwater well at South China Sea. Undoubtedly, the analysis of downhole hydrostatic pressure is a key process in drilling engineering operations. Too often, failure to accurately monitor wellbore ESD can lead to high risk activities, such as excessive well costs and unscheduled trouble time. Consequently, the HT/HP density model proposed in this paper, which provides an effective way to deal with those problems, is worth applying and promoting.