The highly luminescent blue fluorescent carbon quantum dots (CQDs) were prepared via the one-step hydrothermal method using L-ascorbic acid (LAA) as a carbon source. The obtained carbon quantum dots (LAA-CQDs) demonstrated high sensitivity and selectivity for detecting Cu²⁺ and Hg²⁺ based on the efficient fluorescence quenching effect, with the lowest limits of 7.59 nM and 2.29 nM, respectively. Moreover, LAA-CQDs based paper sensors were fabricated, which showed a clear fluorescence response after being exposed to Cu²⁺ and Hg²⁺, with visual detection limits of 5 μM and 3 μM, respectively. The cyclic utilization of the paper sensors was realized by restoring their fluorescence after soaking them in saturated ethylene diamine tetraacetic acid solution. The sensors can be recycled to detect Cu²⁺ and Hg²⁺ ions for at least four turns, which was conducive to environmental protection and saving resources. The LAA-CQDs with good environmental applicability have been successfully applied for the detection of Cu²⁺ and Hg²⁺ ions in real environmental water samples. In addition, we verified the low toxicity and good biocompatibility of LAA-CQDs via cytotoxicity experiments and cell imaging.