Abstract: To investigate deformation and fracture behavior of rock mass under the action of side-discharging, unstable failure process under the action of side-discharging is reproduced by using discrete granular flow software of PFC^2D. Firstly, the parameters of rock mass for PFC^2D calculation are obtained by uniaxial compression test of rock similar material, and then based on it, numerical simulation test of deformation and fracture under the action of side-discharging is conducted. Research results show that the main failure behavior of rock mass presents as two conjugated tension-shear main fractures which are interrelated under the action of side-discharging. Crack expansion can be divided into five stages. Rock deformation of side-discharging shows strong dilatation or swelling in the direction of side-discharging. Finally, three blocks of rock mass will occur mutual diastrophism along the main fractures plane. The volume expansion value of rock mass shows exponential decreased relation with unloading rate, and the faster the unloading rate is, the more obvious the brittle characteristics are, which is burstiness, violent and complexity.