The sodium super ionic conductor (NASICON) has been rapidly developed as an electrolyte for secondary batteries owing to its high ionic conductivity at low temperatures. However, it is very challenging to develop a proton conductor with good conductivity at an intermediate temperature range. Herein, a promising proton conductor can be obtained in NASICON materials, such as Li1+xSrx/2Zr2-x/2(PO4)3 (x = 0.5, 1.0, 1.5, and 2.0). The feasible migration of lithium ions leads to the formation of abundant vacancies for fast proton transfer. The cell based on the Li2.5Sr0.75Zr1.25(PO4)3 electrolyte exhibits an excellent peak power density of 742.85 mW cm–2 at 550 °C. Optimizing the electrode–electrolyte interface can further improve the electrochemical performance. We observed Li+ and proton mixed conductivity in NASICON at medium and low temperatures. The protons are in situ intercalated into the lithium vacancies in the NASICON material through the lithium-ion/proton exchange mechanism and are transported by interconnecting interstitial lithium vacancies.