In present letter, based on density functional theory plus dispersion (DFT-D) and a self-consistent charge density-functional tight-binding (SCC-DFTB) method, the structural and electronic properties are reported, and the phase transition are investigated by analyzing its thermodynamics properties and IR spectrum of RDX. The anisotropy of alpha- and gamma-RDX were discussed at 0-10 GPa. By fitting the third-order Birch-Murnaghan equation of states, the bulk modulus and its pressure derivative of RDX were determined. The alpha-RDX phase is found stable at ambient condition, however, under pressures, both the values of lattice constants a, b, c and the Delta E-vdw at around 4 GPa show abrupt changes which indicate a structural transition occurred. By analyzing the linear compressibility of a, b, c axes at 0-8 GPa, one clearly see that the molecules in alpha RDX phase underwent rotations and translational motion to their position in the gamma-RDX phase at about 4 GPa, which validates the alpha-gamma phase transition. The IR spectra of alpha-form and gamma-form RDX was calculated by analyzing the trajectory of molecules motion, which also show the phase transition from the spectra changes. Employing the quasi-harmonic Debye model, the enthalpy and specific heat were investigated at various pressures of both phases. The condition of equal enthalpies in both phases also indicates the phase transition of alpha-form to gamma-form at around 4 GPa. The variation of specific heat with temperature approaches to the classical Dulong-Petit's law at high temperature, while at low-temperature it obeys the Debye's T-3 law. (C) 2015 Elsevier B.V. All rights reserved.