A flexible potential containing both intra- and intermolecular potential was used to calculate pressure dependence of phonon and vibron frequency shifts of two polymorphs of Pentaerythritol Tetranitrate (PETN) at the pressure ranging from 0 to 3.0 GPa. The intramolecular potential contains bond stretching, angle bending, torsional and non-bonded motions terms. The intermolecular potential used is of the Buckingham 6-exp form plus charge-charge Coulombic interactions. The flexible potential can predict accurately lattice parameters, lattice energies and elastic constants. Predicted mode Grüneisen coefficients are in the range from 1.88 to 4.09 for the modes in phonon region (ω≦110cm-1) of PETN I, and from 1.5 to 3.19 for PETN II. The averaged Grüneisen coefficients of the modes in phonon region is 2.94 and 2.69 for PETN I and PETN II, respectively. In PETN I and PETN II, mode Grüneisen coefficients are less than 1.55 for the modes with the frequency of 110 - 550 cm-1, and less than 0.2 for the modes below 550 cm-1.