Abstract: Low rock breaking efficiency is one of the drilling difficulties in deep and ultra deep layers. Due to the influences of temperature and pressure in deep layers, rock breaking efficiency in deep and ultra deep layers is more different from that in shallow layers. In this paper, the influences of temperature and pressure on rock’s equivalent drillability grade was tested and analyzed based on high temperature and high pressure drilling simulator. In addition, sensitivity analysis was carried out on the influential factors of rock breaking efficiency by performing rock breaking experiments in the environment with the temperature of 20 ℃, 150 ℃ and 300 ℃ and the confining pressure of 25 MPa and 50 MPa. It is experimentally indicated that under the action of one single factor (temperature or pressure), granite’s equivalent drillability grade decreases with the increase of temperature, but increases with the increase of pressure. Under the coupling action of temperature and pressure, granite’s equivalent drillability grade is influenced more by the factor of pressure than the temperature and it is increased by 1-2 levels. Under the temperature of 20-300 ℃, rock breaking efficiency increases with the increase of temperature and the sensitivity of rock breaking efficiency to temperature increases at high weight on bit (800 N) and decreases at high rotation speed (50 r/min). When the confining pressure is in the range of 0-50 MPa, rock breaking efficiency decreases with the increase of confining pressure. In the environment with the temperature of 150 ℃ and the confining pressure of 50 MPa, rock breaking efficiency is positively correlated with weight on bit and rotation speed and it is approximately in a linear relation with rotation speed. Rock breaking efficiency is more sensitive to weight on bit and rotation speed than temperature and confining pressure. To sum up, when selecting or designing bits based on rock drillability grade to drill high temperature and high pressure strata, it is necessary to take into consideration the influences of temperature and pressure while reinforcing the drilling parameters, so that rock breaking efficiency can be improved economically and efficiently.