Abstract: To prevent the occurrence of complicated situations, the effect of temperature on borehole wall stability is studied. Inthis paper, the law that the circumferential effective stress, the rock mechanical properties and the drilling fluid density changed withtemperature is discussed. It is found that the change of temperature could result in thermal stress, and thus enable the Biots factors tochange. As a result, the distribution of circumferential effective stress is changed. As the temperature increases, the rock mechanicproperties get poor. Thermal cracking may occur, which ultimately results in lost circulation and the collapse of wells. Temperatureand pressure play a role on HTHP wells at the same time, but the temperature effect is more obvious, which will result in the decreaseof ECD. On the basis of the above analysis, it is concluded that temperature has a major impact on the security of ultra-deep formationdrilling. So it is critical to minimize the number of tripping and keep the drilling fluid circulation during the course of drilling. This isuseful for determining a reasonable safe density window and imported density of drill fluid.