The investigation of carbon materials with nanostructure is of great importance from both scientific and technological points of view due to their unique properties and promising potential application in many areas. As a novel fluorescent nanomaterial, carbon dots (CDs) show their superior characteristics such as outstanding water solubility, facile functionalization, low toxicity, favorable biocompatibility, high sensitivity and selectivity to target analytes and excellent photostability, and have promoted a great deal of interest in developing bio-imaging, medical diagnose, chemical sensors and photovoltaic devices. In general, chemical doping with N or S element aiming to tune compositions and structures of CDs is considered very effective to intrinsically modify the properties of carbon materials. In this work, we chose a simple combination with citric acid and cysteamine to prepare N,S co-doped CDs (NSCDs) via a same simple hydrothermal synthesis. The effect of hydrothermal temperature on photoluminescent property and quantum yield of the as-prepared NSCDs in both solution and the solid state was investigated. Based on the detailed characterization, the structure and possible photoluminescence mechanism of the NSCDs were discussed. The NSCDs synthesized at 200℃ show significant quantum yield (80.5%). Specially, the obtained NSCDs can exhibit strong blue or yellowish-green fluorescence in solid state as well as blue fluorescence in aqueous state. Besides, multifunctional applications of the as-prepared NSCDs such as selective detection of both anions and cations, preparing fluorescent polymer films and cell imaging have been demonstrated