One of the problems that exist in the secondary sodium ion battery is the low working temperature of the electrolyte, which makes it easy to explode when exposed to free air. The type of solid electrolyte that can be used in high temperatures is NASICON (Natrium Super Ionic Conductor). In this study, magnesium was added to the NASICON structure to increase the ion conductivity of the solid electrolyte as a sodium ion-based battery electrolyte material. The synthesis method used is the solid-state reaction method by mixing sodium carbonate, silicon dioxide, zirconium oxide, ammonium dihydrogen phosphate, magnesium oxide and some anhydrous ethanol into a ball mill for 12 hours, dried at 80°C for 12 hours then calcined at temperature 1125 ° C for 12 hours with a heating rate of 2 ° C minute-1. Then the material is pressed at 15MPa to produce pellets with a diameter of 10mm and the sintering process is carried out at a temperature of 1175 ° C for 12 hours at a speed of 1 ° C minute-1. The doping used varies from 0 to 5mol% of magnesium. XRD results showed that all variations showed a rhombohedral phase. The highest ionic conductivity value was 8.59 x 10-4 S.cm-1 with a magnesium doping variation of 2mol%.