Within the diamond anvil cell, the Raman spectroscopic study of phengite was developed at room temperature and under various pressures from 1 atmp to 20 GPa. The authors analyzed the correlation of the modes 266, 708 and 3618 cm-1 with the variation of pressure. The mode of 708 cm-1 increases linearly with the increase of pressure, i.e., y (Raman shift, cm-1)=0.5238x (pressure, GPa) +712.31, and R2 (correlation coefficient)=0.9656, and the mode disappears at 4.7 GPa as a result of substituting Si for Al. The mode of OH- band and the intensity decrease linearly with the increase of pressure, i.e., y=-0.3402 x+3617.8 and R2=0.9662, and the mode disappears at 18 GPa, which is assumed to be the maximum pressure for the dehydration of phengite at room temperature.