Abstract: High risks exist in drilling and completing HPHT wells. This paper estimates the effects of formation temperature gradient, inlet mud temperature and mud types on the equivalent static density (ESD), and presents the calculation model of ESD under HPHT environment. In addition, the effects of ESD variations on wellbore static pressure, flowing pressure, equivalent circulation density (ECD), surge and swab pressure are also investigated. The results show that errors resulting from ignoring variations in mud properties are small in relative shallow wells, however, in HPHT wells, especially in those wells with narrow safe density margins available, changes in mud density due to elevated temperature and pressure and its effects on wellbore pressure system must be taken into account to ensure safe drilling and completion.