A novel hemicyanine-based fluorescence turn-on chemodosimeter for Hg²⁺ by mercury triggered hydrolysis of the vinyl ether group has been reported. The probe has the unique advantages of easy-preparation, good water solubility, excellent selectivity, high sensitivity, and fast response time (∼60 s) towards Hg²⁺ in aqueous solution. Furthermore, the probe is demonstrated to be qualified to detect Hg²⁺ in living cells.