Nitrocellulose (NC), a highly flammable material, has caused multiple runaway explosions worldwide. Although the thermal stability of NC blended with incompatible materials has been extensively studied, no research on the suppression of spontaneous combustion by using a humectant has been conducted. Ionic liquids possess superior characteristics and have been employed as flame retardants. In this study, 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) was selected as a humectant of NC. First, a series of experiments were conducted to determine the feasibility of the study. Scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy were used to investigate the microstructure, nature, and functional groups of [Bmim][PF6] that affect NC. Next, the inhibition of [Bmim][PF6] on NC was confirmed through an intuitive heating test. Subsequently, the thermal stability of three types of NC systems was evaluated, and the action of [Bmim][PF6] was analysed. Thermokinetic parameters were analysed using differential scanning calorimetry, and the apparent activation energies were evaluated using the Starink method and Friedman method. The time to maximum rate under adiabatic conditions and self-accelerating decomposition temperature were calculated using simulations. Finally, the decomposition of the NC systems was analysed using thermogravimetry-infrared spectroscopy, which elucidated the reasons underlying [Bmim][PF6]-induced inhibition of the spontaneous combustion of NC.