Abstract: In order to study the unloading mechanical characteristics of deep rock under high pore pressure,the sand-stone specimens were prepared,and the triaxial compression test and unloading test under different initial unloading levels and pore pressures were performed in laboratory. Stress-strain characteristics,strength deformation characteristics and constitutive model under unloading were studied. The test results show that for unloading test,when the initial un-loading level n decreases or the pore pressure p increases,the ultimate strength and axial strain of sandstone decrease, showing more obvious brittle characteristics,and with the increase of lateral strain,the dilatancy characteristics are more obvious. Compared with the conventional triaxial compression test,the peak cohesive c of the unloading test is re-duced by 59. 78% and the peak friction angle is increased by up to 54. 72% . With the initial unloading level of n de-creased by 0. 1,the peak c( φ) decreases by approximately 10% (10% -15% ). The unloading stress-strain curve is divided into 4 sections,and the Mohr-Coulomb is chosen as the yield criterion. The parameters of af ,ar ,bf and br are fitted by unary and binary function,and the variation law of each parameter is obtained. Finally,the unloading constitu-tive relation is derived. And the results show that the theoretical model is in good agreement with the experimental re-sults.