In this study, a near-infrared, indole-cyanine probe, i.e. Cy5-HD, was designed for the detection of ascorbic acid for the first time. The probe has a hydrazone moiety that can be hydrolyzed by Cu²⁺ to induce the fluorescence quenching of Cy5-HD. However, the fluorescence quenching ability of Cu²⁺ will be weakened or even disabled in the presence of ascorbic acid, thus the fluorescence recovers. The practical applicability of Cy5-HD was tested by the determination of ascorbic acid in human urine samples, and satisfactory results were obtained.