SnS nanoparticles were synthesized with a facile hydrothermal method and characterized by X-ray diffraction (XRD), Raman, transmission electron microscope (TEM) and scanning electron microscope (SEM). The red light photoresponse of the SnS-based devices in different gas environments were also systematically investigated, and revealed that the adsorbed gas molecules play important roles in the photosensitive properties. Compared with that in vacuum, the photosensitivity was enhanced in O₂ (or air) and reduced in NH₃. The dynamic response time was much longer in a gas environment. These influences were ascribed to the charge transfer between the adsorbed gas molecules and SnS.