By using magnesium carbonate tri-hydrates (MgCO₃·3H₂O) as starting material, anhydrous magnesium carbonate (MgCO₃) was successfully synthesized by dehydration method under high pressure for the first time. The property of as-synthesized sample was investigated by powder X-ray diffraction (XRD) and Raman spectroscopy, respectively. The high pressure synthesis was performed at the various conditions to ensure that the optimal condition is 3 GPa and 800 °C for 1 h, and it is reasonable to explain the principle of MgCO₃ synthesis under high pressure. The size of micro-particles observed from Scanning Electron Microscope (SEM) image exceeds to 100 μm. As a good and important flame retardantmaterial, its thermal property is investigated by Thermogravimetric (TG) analysis, Differential Scanning Calorimeter (DSC) measurement and high temperature X-ray diffraction (XRD) refinement, and the endothermic heat (128.59 KJ/mol), the thermal expansion coefficient along c-axis (α_c = 1.639 × 10⁻⁵/°C), the thermal expansion coefficient along the a-axis (α_a = 4.632 × 10⁻⁶/°C) and the volume thermal expansion coefficient (α_V = 2.576 × 10⁻⁵/°C) was quantified.