The phase transitions of calcite at high temperature and high pressure were investigated by using hydrothermal diamond anvil cell combined with Raman spectroscopy. The result showed that the Raman peak of 155 cm-1 disappeared, the peak of 1 087 cm-1 splited into 1083 and 1 090 cm-1 peaks and the peak of 282 cm-1 abruptly reduced to 231 cm-1 at ambient temperature when the system pressure increased to 1 666 and 2 127 MPa respectively, which proved that calcite transformed to calcite-Ⅱ and calcite-Ⅲ. In the heating process at the initial pressure of 2 761 MPa and below 171 ℃, there was no change in Raman characteristic peaks of calcite-Ⅲ. As the temperature increased to 171 ℃, the color of calcite crystal became opaque completely and the symmetric stretching vibration peak of 1 087 cm-1, in-plane bending vibration peak of 713 cm-1 and lattice vibration peaks of 155 and 282 cm-1 began to mutate, showing that the calcite-Ⅲ transformed to a new phase of calcium carbonate at the moment. When the temperature dropped to room temperature, this new phase remained stable all along. It also indicated that the process of phase transformation from calcite to the new phase of calcium carbonate was irreversible. The equation of phase transition between calcite-Ⅲ and new phase of calcium carbonate can be determined by P(MPa)=9.09T·(℃)+1 880. The slopes of the Raman peak (ν1 087) of symmetrical stretching vibration depending on pressure and temperature are dν/dP=5.1(cm-1·GPa-1) and dν/dT=-0.055 3(cm-1·℃-1), respectively.