As a rising star in the field of quantum dots, carbon quantum dots (CQDs) have attracted increasing advertence in recent years. In this work, sodium sulfide (Na2S) was used as the sulfur source for the first time to synthesize sulfur-doped carbon quantum dots (S-CQDs) with stable blue fluorescence by one-step hydrothermal. These synthesized S-CQDs have a uniform size of about 4.2 nm. Compared with CQDs, the prepared S-CQDs showed higher fluorescence intensity. And it also exhibited excitation-independent behavior, relatively high fluorescence quantum yield (QY = 21.1%) and outstanding photobleaching resistance and stability. More importantly, the S-CQDs was used as a novel fluorescent sensor to detect Hg2+ with high sensitivity and selectivity. Benefitting from above merits, this work provided a new strategy to improve the fluorescence intensity of CQDs and as-prepared S-CQDs showed the potential applications in the detection of heavy metals in the environment.