[en] Nitrogen-sulfur co-doped carbon quantum dots (N, S-CQDs) with good photoluminescence properties were prepared by hydrothermal method using citric acid (CA) and methionine (Met) as precursors. Co-doping with N and S facilitates the electron transfer rate and coordination interaction between N,S-CQDs and Fe(III) ions which acts as a quencher of fluorescence. Based on a simple redox principle, a highly sensitive and selective method for the detection of ascorbic acid (AA) and H₂O₂ was successfully developed. The calibration curves obtained are linear for the current versus AA and H₂O₂ concentration over the range 50–500 μM and 10–140 μM, respectively. And the detection limits for AA and H₂O₂ are 4.2 μM and 1.9 μM, respectively. The quantitative analysis of AA and H₂O₂ in various juices and H₂O₂ disinfectant with Fe³⁺/CQDs or Fe²⁺/CQDs gave the recoveries of 87.8%–117.5% and 99.2%–106.4% with relative standard deviations (RSD) of 1.6–5.1% and 1.7–3.1%, respectively, showing satisfactory results for the determination of AA and H₂O₂ in actual application. The proposed strategy may provide a new pathway to developing inexpensive and sensitive way for the detection of various redox reaction–involved system.