SnO₂ nanofiber/nanosheets with hierarchical nanostructures were successfully synthesized via a facile hydrothermal method by using hollow SnO₂ nanofibers as the backbone. The microstructure, morphology, chemical composition, oxidation states and surface areas of SnO₂ nanofibers, SnO₂ nanofiber/nanosheets and SnO₂ nanosheets were comparatively studied by X-ray diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), Transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), and Brunauer-Emmett-Teller (BET). The characterization results indicated that the hierarchical SnO₂ nanofiber/nanosheets were constructed by nanosheets arrays uniform growth on the surface of nanofibers. Sensing performances of SnO₂-based nanomaterials were investigated utilizing formaldehyde (HCHO) as a target gas. Compared to SnO₂ nanofibers, nanosheets and the physical mixture of nanofibers and nanosheets, the gas sensor based on SnO₂ nanofiber/nanosheets exhibited better response, more excellent selectivity, transient response and trace detection ability to HCHO gas. The response (R_a/R_g) of the gas sensor is 57 toward 100 ppm HCHO at 120 °C, which is about 300% and 200% higher than that of pure SnO₂ nanofibers sensors and SnO₂ nanosheets sensors, respectively. Furthermore, the sensor has an excellent response/recovery performance with 4.7 s and 11.6 s for detecting 100 ppm HCHO. Both the growth process and the gas sensing mechanism of SnO₂ hierarchical nanostructures were discussed. Successful preparation of SnO₂ nanofiber/nanosheets is attributed to uniform decoration of seeds on the nanofibers and suitable growth conditions of nanosheets. Enhanced sensing performance mainly result from the synergistic effect of nanofibers and nanosheets, hierarchical structures and larger specific surface areas. The synthetic strategy can also be applied in preparing hierarchical materials of different constituent materials.