Garnet-type oxide solid electrolytes are the critical materials for all-solid-state lithium ion batteries. Nanoscale spectroscopic analysis on solid electrolytes plays a key role in bridging the gap between microstructure and properties. In this work, Auger electron spectroscopy (AES), which can directly detect lithium element and distinguish its valence state, was applied to characterize the garnet-type Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO). Different spectroscopy parameters were evaluated and optimal acquisition conditions were provided. Electron induced precipitation of lithium metal from LLZTO was observed. By exploring the influence factors of precipitation and combining transmission electron microscopy (TEM) and focused ion beam (FIB) experiments, the underlying mechanism of the phenomenon was revealed and previous controversy was resolved. The analysis method was also extended to other types of solid electrolytes, and this work provides a reference for future in-depth research on the structure–property relationship of solid electrolytes using AES.