The solid-state electrolytes are of great interest and importance due to their improved safety. For building the solid-state sodium-metal batteries, Na3Zr2Si2PO12 (NZSP) represents a promising candidate as it features high chemical stability against air exposure and a high Na+ conductivity. The NZSP pellets are usually calcined at a high temperature, and the high volatility of Na and P elements easily lead to the formation of impurity phase. In this work, we have studied the eff ects of calcination temperature and stoichiometry on the phase purity and ionic conductivity of the NZSP electrolyte. For the stoichiometry, the introduction of 5 mol% excessive P results in formation of more Na3PO4 and glass-like phase at the grain boundary, and eff ectively suppresses the Na dendrite growth, then accounts for improved cycling performance of Na||Na symmetric cell. Our work provides insights on reasonable design and preparation of NZSP electrolyte towards practical realization of solid-state Na-metal batteries.