Abstract: By means of the improved SHPB with axial pre-pressure and confining pressure, experimental studies on the dynamic mechanical properties of sandstone under different unloading confining pressures are performed. In the experiments, the sandstone specimens are subjected to the static-dynamic coupling loads with fixed axial static pressure and impact energy and various rates of unloading confining pressure. The effects of unloading rates of confining pressure on rock dynamic strength and damage are investigated. The results show that when the unloading rates vary in the range of 0.5-10 MPa/s, the dynamic strength and energy density of sandstone gradually decrease, but the fractal dimension gradually increases. However, when the unloading rate reaches 200 MPa/s, the dynamic strength and energy density increase, and the fractal dimension decreases.