NASICON-type solid-state electrolyte is characterized by high electrical conductivity but its application in all-solid-state battery is limited by the high sintering temperature and poor interface contact with the electrodes. Here, solid-state reactive sintering, without intermediate calcination and ball-milling steps and no sintering additive, is proposed to prepare dense and highly conductive NASICON at lower temperatures. The samples sintered at 950 and 1000 °C achieve relative density of ~90% and high ion conductivity of 8.43 × 10−4 and 1.48 × 10−3 S cm−1 at room temperature, respectively. The reasonable interface contact between sodium metal and 950 °C-sintered electrolyte affords the symmetric sodium battery to cycle stably at 0.05 mA cm−2 for ~1000 h and full battery at 0.1C (0.02 mA cm−2) at room temperature. This work provides a new strategy to prepare NASICON solid-state electrolyte, which can be extended to prepare other solid-state electrolytes and thus promote the development of all-solid-state battery.