Abstract: The safety pressure window is narrow in deep well with HPHT conditions,the differential pressure and differential temperature at different depth of wellbore are very huge;at the same time,the density of the fluid in the well would change when expanding due to formation heat and pressure of fluid column;therefore the improper selection of initial density of in-well fluid would lead to pressure instability.With independently developed HTHP fluid density measuring apparatus,the experiment was conducted to test the influence of temperature and pressure on fresh water,spacer fluid,water mud and mineral oil density and relevant curves were obtained;the proper density model for temperature and pressure change in well cementation was selected—Dodson-Standing Model;With Drillbench software,the water-based and oil-based drilling fluid with initial in-well density of 1.2 kg/L and 2.0 kg/L was calculated,four corresponding curves,including static equivalent density variation curve were acquired under different geo-thermal gradient.These curves could serve as reference to the density design of drilling fluid and cement slurry for deep wells under HTHP conditions.The experiment results indicated that,in the wells deeper than 5000 m with BHST higher than 270 ℃, the density of water-based fluid could be decreased by 5.58% and that of oil-based fluid decreased by 6.41%,compared with the in-well density of 1.2 g/L.