Abstract：Sandstone samples are selected by the compressive wave velocity and the shear wave velocity. Then test scheme has been designed to simulate rock mechanical behaviors under the altering pore pressure of high- pressure oil and gas reservoir according to the geological structure. All the triaxial tests have been performed on the triaxial compression test system GCTS–1000 with high temperature and high pressure. The test results show that：rock strength doesn¢t increase monotonically with confining pressure while the internal pore pressure is increasing and confining pressure keeps constant；but the rock strength will decrease with the net increasing confining pressure first and increase abnormally then. The minimum value appears when the geostress gradient is equal to 2.20 MPa/(100 m)；but the sandstone compressive strength grows up with the increase of geostress gradient. The relationship curve between rock strength and pore pressure is V-shaped. The relationship curve between elastic module and pore pressure appears as a wave shape and the maximum difference among elastic module is 2 909 MPa. In the experiment，the Poisson¢s ratio increases gradually and approaches 0.5 when the geostress gradient coefficient is larger than 2.00 MPa/(100 m). This particular phenomenon of sandstone triaxial strength has been observed during repeated tests. The test result is significant to simulate the true three- dimensional deformation and failure of the system composed of high-pressure gas and oil reservoir，cement sheath and casing.
