Abstract: Based on the RFPA numerical method and the parallel technology, a microscopic hydro-mechanical coupling model to reflect the process of rock damage is established. For a square rock model with 1200000 elements, 3D scientific computation is performed during the process of hydraulic fracturing under 4 different stress states. The results show that the initial pressure doesn't coincide with the buckling pressure, and that the extension form, surface planeness, tendency, extension instability process which is tensional and spatial distribution shape of cracks are influenced by the stress states. Cracks are distributed in the form of shaft film when the maximum principal stress direction is vertical, if the horizontal stress difference is bigger, instability is faster, and the crack surface is plane. Cracks are distributed in the form of shaft film when the minimum principal stress direction is horizontal. Cracks are always distributed in the minimum stress direction plane under different principal stress situations. There is competition trend between initiation location and propagation direction when the 3D principal stresses are equal, and there is no laws in space distribution and cracks are branched. Numerical simulation results are in accordance are with physical experimental ones. The study is valuable to the engineering design of hydraulic fracturing.