In-situ synchrotron X-ray diffraction and Raman spectroscopy were used to study the stabilities, thermal expansion and vibrational modes of synthetic Mg-3(PO4)(2). The polymorphs (Mg-3(PO4)(2)-I, II, III) were investigated in the temperature range of 299 similar to 1173 K at ambient pressure. An irreversible phase transition was observed for both Mg-3(PO4)(2)-II and Mg-3(PO4)(2)-III, whereas Mg-3(PO4)(2)-I is stable in the present study. Based on the in-situ synchrotron X-ray diffraction and Raman spectroscopic measurements, Mg-3(PO4)2-II and Mg-3(PO4)(2)-III transform to Mg-3(PO4)(2)-I at 1073 K and 1023 K, respectively. The volumetric thermal expansion coefficients of Mg-3(PO4)(2)-I, II and III were determined as 3.31(4) x 10(-5) K-1, 3.91(4) x 10(-5) K-1, and 3.25(5) x 10(-5) K-1, respectively. All three Mg-3(PO4)(2) polymorphs show axial thermal expansive anisotropy since the thermal expansion coefficients along different axes are inconsistent. The effect of temperature on the Raman vibrations of the three Mg-3(PO4)(2) polymorphs was quantitatively analyzed. And the isobaric mode Grfineisen parameters of three Mg-3(PO4)(2) polymorphs are calculated, which are in the range of 0.07 similar to 3.54.