The influence of diphenylamine (DPA) on the thermal stability of nitrocellulose (NC) was investigated by isothermal decomposition kinetics. Pressure versus time curves of the gas generated by the thermal decomposition of NC/DPA (3%) mixed sample at 378.15-398.15 K were obtained by using the isothermal decomposition gas metering device. There is a visible inflection point in this curve. The thermal decomposition kinetic parameters of NC/DPA (3%) composite were calculated by Arrhenius equation and model-fitting method. The storage life of NC and NC/DPA composite was estimated using Berthelot equation. The results show that the activation energy of NC/DPA (3%) before the inflection point is 164.6 kJ·mol^-1, and the activation energy of NC/DPA (3%) mixed sample after the inflection point is 150.4 kJ·mol^-1. Compared with the NC raw material, the activation energy of NC/DPA (3%) before the inflection point is increased by 17.7 kJ·mol^-1. With a decomposition extent of 0.1% as the criterion, the storage life of NC/DPA (3%) at room temperature is 18.3 years, indicating that the storage life of NC can be extended by 7.7 years after the addition of 3%DPA.