In this work, SnO2 nanoflowers were prepared by a simple one-step hydrothermal process. The morphology and structure of SnO2 nanoflowers were characterized by SEM, TEM, Raman spectroscopy, and XRD, which demonstrated the good crystallinity of the SnO2 tetrahedron structure of the as-synthesized materials. In addition, the sensing properties of SnO2 nanoflowers were studied in detail. It was found that the SnO2 nanoflower-based gas sensor exhibits excellent gas response (9.2 to 120 ppm), fast response and recovery (2/15 s to 6 ppm), good linearity of correlation between response (S) vs. concentration (C) (lgS = 0.505 lgC − 0.147, R2 = 0.9863), superb repeatability, and selectivity at 300 °C. The outstanding performance can also be attributed to the high specific surface area ratio and size of SnO2 nanoflowers close to the thickness of the electron depletion layer that can provide abundant active sites, promote the rate of interaction, and make it easier for gas molecules to diffuse into the interior of the material. Therefore, SnO2 nanoflowers can be an ideal sensing material for real-time monitoring of low-concentration HCHO.