The invention discloses preparation and application of a water-soluble fluorescence sensor for ratio checking of cysteine. The fluorescence sensor is a novel water-soluble fluorescence sensor which is prepared from a carbon quantum point, fluorescein isothiocyanate and acryloyl chloride as raw materials prepared according to the prior art and is used for implementing ratio checking of cysteine. By adopting the fluorescence sensor, high-selectivity and high-sensitivity ratio checking on cysteine can be achieved in a pure-water solution. Compared with the conventional fluorescence detection technique, the fluorescence sensor disclosed by the invention has advantages of being high in cysteine selectivity, rapid in response to cysteine, low in cytotoxicity, excellent in water dispersibility, relatively low in investment cost, simple in synthesis route and the like, is applicable to amplified synthesis and practical production application and has great application prospects in technical fields such as analytic chemistry and life sciences.