High-pressure Raman scattering studies are performed on acetonitrile in a diamond anvil cell up to 24.8 GPa at room temperature. The results show that liquid acetonitrile transforms into β phase at 0.3 GPa, and then into α phase at 0.8 GPa. With increasing pressure, the α to γ phase transition is observed at 9.8 GPa which results from the rearrangement of molecular structure. After further increasing the pressure to 21.5 GPa, the external Raman modes of acetonitrile completely disappear, and the result suggests that acetonitrile ultimately turns into an amorphous state accompanying the decreases of transmittance light. The high pressure behaviors of polar molecular acetonitrile will provide important information for the study of lattice dynamics of molecular crystals.