Abstract: Solid rocket motor filled with high energetic propellant may cause fire or explosive hazards when falling and impacting the earth. In order to investigate the explosive reaction characteristics of solid rocket propellant under low impacting velocity, the numerical model of solid rocket motor impacting on the target was set up. In the models, a coupled thermal-mechanical algorithm was adopted to describe the energy conversion from mechanical to internal energy. The heat generation rate of propellant decomposing before time to explosion was modeled by Arrhenius equation. The process of the solid rocket motor with diameter of 200 mm and 480 mm radically impacting was numerically simulated. The thresholds speed was obtained and agreed with the experimental results. The results indicate that the critical speed of solid rocket motor firing is related to the mass of solid motor. Large motors are more vulnerable at low impact velocities than for small motors. The critical speed for ignition is linear with logarithm of mass of propellant.