Thermal decomposition of a nitroguanidine-based propellant at different pressures was investigated under non-isothermal conditions by high-pressure differential scanning calorimetry (PDSC). It was found that only one visible exothermic peak appeared in DSC curves at the pressure. With the increase of the environment pressure, the peak temperature decreased, and the decomposition heat increased. Application of elevated pressures changed the mechanism of thermal decomposition of the nitroguanidine propellant as compared with that at 0.1MPa. The reaction followed the sigmoidal model, where n=0.97 (0.1MPa), or n=0.96 (high pressures). The mechanism of the process at 0.1MPa can be classified as chemical reaction, and the kinetic equation can be d/dt=1027.53; whereas at high pressures, it shifts to nucleation and growth, with d/dt=1023.99(1-)e-2.59x104/T at 2MPa and d/dt=1022.39(1-)e-2.42x104/T at 4MPa. Pressure increase lowered the decrease in critical temperature, and lowered entropy of activation (S), enthalpy of activation (H), free energy of activation (G), as well as the thermal safety of the nitroguanidine propellant.