Carbon dots (CDs) have been studied for years as one of the most promising fluorescent nanomaterials. Herein, fluorescent CDs excited by visible light emitted red fluorescence with a quantum yield of 32.8% via a facile hydrothermal treatment. In addition to being applied in cell imaging, the as-prepared CDs were also utilized in the detection of ammonia and formaldehyde in aqueous solutions because of their pH-dependent properties, with a low limit of detection of 3.7 nM and 44 μM, respectively. Furthermore, we synthesized flexible and transparent fluorescence films by mixing the as-prepared CDs and PVB powder, which can be utilized to directly detect ammonia gas under visible light according to significant colour changes. Therefore, the as-prepared CDs would have more prominent and influential application in clinical diagnostics and environmental analysis.