Stimulated by the extensive application and research value, the study of anhydrous magnesium carbonate (MgCO3) has been a subject of great concern recently, so that a basic problem in designing a method of effectively synthesizing MgCO3 is very worth considering. In previous studies, different methods were reported to synthesize MgCO3 successfully but they still have some obvious deficiencies. The micro-particle sizes are too small to satisfy the basic requirements of micro-analysis. Thus, it is needed to explore the new methods of artificially synthesizing MgCO3 with the simple process and the high efficiency. By using magnesium oxalate dihydrate (MgC2O42H2O) as starting material, MgCO3 sample is successfully synthesized by a solid reaction under high temperature and high pressure for the first time in this work. The properties of as-synthesized sample are investigated by X-ray powder diffraction and Raman spectroscopy:neither of them shows any impurities existing in the sample. Significantly, the crystallinity quality is greatly improved in the terms of the maximum grain sizes up to 200 micrometers, which could provide a base for MgCO3 single crystal growth in the future. Moreover, compared with the results of previous studies, the reaction time of high pressure synthesis is controlled within 1 h so that the efficiency of the synthesis is greatly improved. Based on thermogravimetric analyses and the results of high pressure experiment under the various pressures and temperatures, the P-T phase diagrams of MgC2O42H2O-MgCO3-MgO at high pressures of 0.5, 1.0 and 1.5 GPa are obtained, and in this case, it is reasonable to explain the principle of MgCO3 synthesis under high pressure strictly. From the P-T diagram, high pressure can greatly improve the thermal stability of material, and the decomposition temperature of MgCO3 obviously increases with pressure increasing. However, due to decomposition temperature of MgCO3 increasing more quickly than that of MgCO42H2O, the stable phase regions of MgC2O42H2O and MgCO3 are separated from each other, and hence, the corresponding temperature and pressure can be controlled to decompose the phase of MgC2O42H2O while stabilizing the phase of MgCO3 so as to obtain MgCO3 successfully. Besides, by using polarizing microscope, the morphology of MgCO3 sample as well as its crystal cleavage plane (1011) is observed clearly, and it is noted that as-synthesized MgCO3 has good optical properties and high-quality crystallinity. The electron probing analysis for MgCO3 thin section is performed to quantify the Mg content and the calculation indicates that the sample composition is Mg0.99CO3.