The present research studied the process of phase transition from calcite-Ⅰ to calcite-Ⅲ under the condition of high hydrostatic pressure using hydrothermal diamond anvil cell and Raman spectrum technique. The hydrothermal diamond anvil cell is the most useful instrument to observe sample in-situation under high temperature and high pressure. The authors can get effective results from this instrument and pursue further research. The method of Raman spectra is the most useful measure tool and it can detect the material according to the spectrum. The result shows that three characteristic Raman peaks of calcite-Ⅰ move to high-position with adding pressure. Water media in system becomes frozen at the pressure of 1 103 MPa, and there is no change in the structure of calcite-Ⅰ. The abrupt change of characteristic Raman peaks of calcite -Ⅰ happens when the system pressure reaches 1 752 MPa, and changed characteristic Raman peaks explain that calcite-Ⅰ changes to calcite-Ⅲ. There are two types of calcite-Ⅲ, and type A happens in the system because of the effect of hydrostatic pressure. The characteristic Raman peak in different areas of minerals shows that the degree of phase transition becomes larger from inner part to edge part. The research also shows the advantage of hydrothermal diamond anvil cell and Raman spectrum for qualitative analysis of mineral structure using in-situ technique.