The SnO₂@ Polyaniline (PANI) nanocomposites were successfully synthesized via a combined approach of hy drothermal and in-situ polymerization. The morphology and structure of the materials were characterized using SEM and TEM. The specific surface area was analyzed by the Brunauer-Emmett-Teller (BET) equation. The results showed that the materials had micro-structure and large specific surface area. The prepared materials were loaded on flexible polyethylene terephthalate (PET) substrate and their gas sensing properties were in vestigated at room temperature. Comparing with PANI sensor, the 20 mol% SnO₂@ Polyaniline (PASn20) sensor exhibited 6.2 times higher response (Rgas/Rair = 29.8) to 100 ppm NH₃at room temperature. Furthermore, the PASn20 sensor possessed the ability to detect low NH₃concentrations of 10–200 ppb and outstanding selectivity. The enhanced sensing performance was attributed to the micro-structure with large specific surface area and the formation of p-n heterojunctions at the surface between PANI and SnO₂.