The solubility of 1-ethyl-3-methylimidazolium hexafluorophosphate ([Emim][PF6]) in methanol under high pressure is newly measured quantitatively according to the correlation between the ratios of Raman intensity and the concentrations. In situ crystallization and cation conformation of [Emim][PF6] from methanol solution under high pressure have been investigated by using Raman spectroscopy in detail. Remarkably, crystal polymorphism was observed and two crystalline phases (phases I and H) coexisted under high pressure up to similar to 1.4 GPa. However, only phase II was obtained by recrystallization at similar to 2 GPa. Our findings may facilitate the development of an effective way for crystallization and purification of ionic liquids under high pressure.