Ab initio phonon calculations on NiAs-type (B8) MgTe are performed to probe the structural behaviors of MgTe under high pressure. We predicted an unstable transverse acoustic mode and a phase transition of B8→orthorhombic Pnma (B31) driven by this soft mode is thus identified. The polymorph of B31 in MgTe adopts MnP-type structure containing chains of metal atoms and is stable above 42.7 GPa through the enthalpy calculations. With the discovery of the B31 phase, the previously theoretical high pressure structures of the tetragonal P4/nmm (PH4I) and the CsCl (B2)are ruled out.