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Improving the density of Na3Zr2Si2PO12 (NZSP) solid electrolyte is crucial for its application in solid-state sodium batteries. Here, bismuth oxide with low melting point has been evaluated as a sintering aid for NZSP solid electrolytes. Research has found that NZSP doped with 5 mass% Bi2O3 increases grain boundary mobility by 3.1 times and has an ionic conductivity of 4.78 × 10−3 S cm−1, with a minimum activation energy of 0.30 eV, approximately 5 times that of undoped Bi2O3. This is because the electrolyte density increased from 77.3 to 93.7%. Compared with the region III process of NZSP without Bi2O3, Bi2O3 promotes the densification process of Bi2O3 dopant coarsening in region III. The constant current cycle of Na | NZSP-5 mass% Bi2O3 | Na symmetric battery indicates that no short circuit will occur within 1200 h. The discharge capacity of solid-state batteries at 0.2 C is 98.5 mAh g−1. This indicates that NZSP doped with 5% Bi2O3 is a good electrolyte choice for sodium-ion batteries. [ABSTRACT FROM AUTHOR]