Ambient-pressure room-temperature superconductivity is one ultimate goal of science, for it will bring worldwide revolutionary changes in all kinds of technology. Several room temperature and near room temperature hydride superconductors at ultra high pressure (⪆100 GPa) have been predicted theoretically. In particular, the hydrogen sulfide (H3S) with T C ≃ 203 K at 200 GPa has soon been confirmed experimentally, establishing a milestone toward room temperature superconductivity. However, high-T C superconductors at lower pressure (≲100 GPa) have not been reported before. In this work, we present high-T C superconductivity of 180 K at a relatively low pressure of 50 GPa in sodium hydride clathrate structure NaH6. The T C can be raised up to 206 K at 100 GPa, similar to the T C of H3S but at a much lower pressure. At 200 GPa, it reaches the highest T C of 210 K, slightly higher than that of H3S. The strong electron-phonon coupling strength given by the T 2g phonon mode at Γ point plays the key role in superconductivity. Our work demonstrates theoretically that hydrides could stabilize at a relatively low pressure and host high-T C superconductivity.