Three dimensional (3D) porous nanostructures assembled by low-dimensional nanomaterial are widely applied in gas sensor according to porous structure which can facilitate the transport of gas molecules. In this work, fish-scale-like porous SnO2 nanomaterials assembled from ultrathin nanosheets with thickness of 16.8 nm were synthesized by a facile hydrothermal route. Then Ag nanoparticles were decorated on the surface of SnO2 nanosheets via a one-step method to improve their gas-sensing performances. The sensing properties of pristine SnO2 and Ag/SnO2 nanosheets were investigated intensively. After decorating with Ag nanoparticles, the characteristics of SnO2 based sensor for triethylamine detection were significantly improved. Especially, the Ag/SnO2 based sensor with Ag content of 2 at% exhibited the highest triethylamine sensing sensitivity at optimum work temperature of 170 °C. The improved sensing properties of Ag/SnO2 sensors were attributed to the sensitizing actions of Ag nanoparticles as well as the unique hierarchical porous architecture.