Starting with the molecular structure of α-RDX, the non-empirical full atom self-consistent force field (SCFF) for the explosive was derived via the first principles including electronic structure theory, intermolecular perturbation theory, and based on the SCFF-phonon approach to calculating the anharmonic free energy, the isentropic locus for α-RDX was obtained, covering a pressure range from the ambient pressure to 55 GPa. The relationships between p(GPa)and V(cm³·mol^-1), and between T(K) and V(cm³·mol^-1) of the isentropic locus are: p=66.59633+4.52375exp(-V/0.68117)+16.56087exp(-V/4.31486)+35.94539exp(-V/32.26906) and lnT=5.06548+0.74774lnV-0.13886(lnV)², respectively. The SCFF and SCFF-phonon methods can be generalized toward other high energetic materials and thus provides an effective theoretical tool for constructing their precise EOSs and computing thermodynamic properties.