We report a facile method for the in situ growth of vertically aligned SnO2 nanosheets on a substrate, which involved the hydrothermal treatment of the substrate covered with electrospun nanofibers of poly(vinyl butyral) containing SnO2 at low temperatures and free of additives. The formation mechanism of nanostructured SnO2 has been explored. Gas sensors based on the composites of SnO2 nanosheets and polyaniline were prepared by vapor phase polymerization of aniline or the dip coating with waterprocessable polyaniline, and revealed good contact between the sensing films and underlying electrode. The nanocomposite sensors demonstrate very high response magnitude toward NH3 (relative resistance change of similar to 3700% towards 10.7 ppm of NH3), ultra-low detection limit (similar to 46 ppb), good sensing repeatability and excellent selectivity at room temperature. Investigations on the gas sensing mechanism of the nanocomposite suggest that the establishment of the p/n heterojunction at the interface of p-type PANI and n-type SnO2 play an important role in enhancing the response magnitude of the nanocomposite. The method could provide new solutions for the direct fabrication of optoelectronic and electrochemical devices based on nanostructured metal oxides and their composites.