In the ignition state, the SRM (solid rocket motor) grain with wall pouring is in the state of superimposed pressure, while the circumferential direction of SRM inner bore is under tensile strain. It is urgent to carry out related research on the failure mechanism of propellant under superimposed pressure. The mechanical behavior of solid propellant under coupling effect of superimposed pressure, temperature and strain rate was investigated through uniaxial tensile tests at different pressure, temperature and strain rates using self-developed superimposed pressure loading system. According to the mechanical response characteristics of the propellant under superimposed pressure, the failure modes of the propellant under superimposed pressure were further studied by means of meso-scale mechanical simulation. At the same time, the morphologic sectional features of the propellant specimens were analyzed by means of scanning electron microscope test. The failure mechanism of the propellant under superimposed pressure was found out by means of combining test and simulation. The results show that the damage interfaces of propellant decrease significantly under superimposed pressure, and with the increasing of superimposed pressure, the damage form of propellant changes from particle dewetting dominated to particle breakage dominated.