The adsorption and decomposition of hexogen (RDX) molecule on the Mg(0001) surface were investigated by the generalized gradient approximation (GGA) of density functional theory (DFT). The calculations employed a supercell (4 × 4 × 4) slab model and three-dimensional periodic boundary conditions. The strong attractive forces between RDX molecule and magnesium atoms induce the RDX’s N − O bond breaking. Subsequently, the dissociated oxygen atoms and radical fragment of RDX oxidize the Mg surface. The largest adsorption energy is −2104.0 kJ mol-1. We also investigated the decomposition mechanism of RDX molecule on the Mg(0001) surface. The activation energy for the dissociation step of configuration V4 is as small as 2.5 kJ mol-1, while activation energies of other configurations are much larger, in the range of 964.9–1375.1 kJ mol-1. Mg powder is more active than Al powder, and Mg powder performs better in increasing the combustion exothermicity of RDX as well.