Two kinds of Ag/CNTs (carbon nanotubes) nanocomposite materials were successfully prepared by silver mirror reaction and hydrothermal methods. Their physical phase, composition, morphology, and structure were characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscope-energy dispersive spectrometry (SEM-EDS). Catalytic effects of the Ag/CNTs nanocomposite on the thermal decomposition of cyclotrimethylene trinitramine (RDX) were investigated by differential scanning calorimetry (DSC). Our results indicated that the irregularly globose nano-Ag particles attached to the surface of the nano CNTs evenly. The nanocomposite product prepared hydrothermally had the highest loading of nano-Ag particles and largest Ag particle size. Both Ag/CNTs nanocomposites influenced the thermal decomposition of RDX, changing the primary decomposition of the liquid phase to a secondary gas phase reaction and resulting in obvious change in the shape of the main decomposition peak. The catalytic effect of Ag/CNTs nanocomposite on the thermal decomposition of RDX was mainly exhibited at lower decomposition temperature.