Abstract: Suitable drilling fluid density is the primary factor for drilling through HT/HP reservoir formation safely and rapidly. But in conventional methods for determining fracturing pressure and caving pressure, the influence of borehole temperature change and drilling fluid penetration is neglected, which would bring in hidden trouble for safe drilling. Thus, according to porothermoelasticity principles, stresses distribution around borehole are determined combined with stresses duo to formation temperature change and drilling fluid seepage. Following formation failure criterion, fracturing and caving pressure calculation mode are deduced when drilling HT/HP reservoir formation, and the influences of temperature change and borehole seepage on safe mud window are analyzed, all of these would be useful for safe drilling fluid density design, when drilling in HT/HP reservoir formation.