Abstract: Two non-reflecting boundary conditions including superposition boundary condition and viscous boundary condition are studied and implemented in Discontinuous Deformation Analysis (DDA) method. The dynamic response of a rock thin bar under the impact of a flying block is simulated, whose boundary condition is fixed, superposition and viscous respectively. Simulation result indicates that, both superposition boundary condition and viscous boundary condition can absorb the reflected stress wave to some extent. The absorbing ability of superposition boundary condition is better than that of viscous boundary condition. The incident stress wave may be less affected when superposition boundary condition is used.