Ceramic-based all-solid-state sodium batteries (Na-ASSBs) are prospective alternative energy storage systems due to the abundant sodium resources and their high safety. A previous study showed that chemical infiltration could methodically improve not only the contact between the positive electrode (Na3V2(PO4)3) (NVP) and solid electrolyte (Na3.4Zr2Si2.4P0.6O12) (NZSP) but also the electrochemcial performance of the battery. However, detailed information on the corresponding structural evolution remains lacking. In this work, the synthesis process of NVP on NZSP substrate was first investigated at the atomic scale by in situ high-resolution transmission electron microscope (HRTEM) and energy dispersive spectroscopy (EDS). The entire calcination process consisted of crystallization and aggregation of NH4VO3 and NaH2PO4 precursors, and then formation of NVP. During the formation process, NaH2PO4 diffused into NVP because of its light atomic weight. In contrast, NH4VO3 maintained its position instead of diffusing. Furthermore, HRTEM movies and the corresponding fast fourier transforms (FFTs) pattern demonstrated that NVP epitaxially grew with NH4VO3 and NaH2PO4. This study first reveals the formation and growth processes of NVP at the atomic scale. These results provide not only fundamental information but also a basis for further development of Na-SSSBs.