Water release in subduction zones is not only an important part of the deep Earth’s water cycle, but also plays an essential role in the physical and chemical properties of rocks constituting the deep Earth. To understand water release processes, it is important to know properties of dehydration in hydrous phases of the downgoing slab. Although it is widely accepted that phengite can be stable to greater depth in subduction environment, behavior of hydroxyl and lattice of it at high temperature and high pressure are less investigated in contrast to other hydrous phases. Here, using IR and Raman spectroscopy, we characterize hydroxyl and lattice of ammonium-bearing and ammonium-free phengite at high temperature and high pressure. No proton transferring and structural phase transition in phengite were observed over the measured temperature and pressure range. Both pressure and temperature induce hydroxyl band shifting to lower frequencies, and pressure has a greater impact. The band width of hydroxyl increases with temperature and pressure. Hydroxyl bond weakening and hydrogen disordering at high temperature and high pressure should be responsible for the spectra variations. On the other hand, the lattice modes soften with increasing temperature whereas stiffen under compression, and ammonium plays an important role in the Grüneisen parameters of the lattice modes, especially the K-O mode. These features of hydroxyl and lattice at high temperature and high pressure could benefit for further understanding dehydration, thermodynamic properties and stability of phengite in subduction zones.