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The structural stabilities and thermal properties of η′ and η phase at high temperature and pressure were investigated via first principles calculations combined with quasi-harmonic Debye approximation (QHA). The calculated results illustrate η′ possess high formation enthalpy with lower structural stability at high temperature, which may transits to equilibrium phase η at temperature range of 510–520 K. The calculated thermodynamic properties indicate η exhibits better thermal stability than η′ at high temperature. The thermoelastic properties of η′ and η were estimated based on the temperature-dependent elastic constants Cij(T), and it is revealed the elastic moduli of η′ are larger than that of η though both two phases become deformable with elevated temperature. Moreover, the evaluated thermoelastic anisotropies suggest η′ and η are essentially elastic isotropic on {0001} plane, while η′ exhibit serious mechanical anisotropy and angular bonding characters on both {10-10} and {01–10} planes at high temperature.