In this work, a simple dichlororesorufin-based colorimetric and fluorescent probe R-Hg with the recognition receptor of carbon- othioate was designed, synthesized, and exploited to determine mercury ions in aqueous solution and living cells as well as in zebrafish. After a series of analytical tests, probe R-Hg showed some excellent characteristics, such as ultrasensitivity, high selectivity, and good water solubility. The probe solution exhibited a very large fluorescence enhancement (about 150-fold) upon addition of He2+. Additionally, probe R-Hg exhibited excellent sensitivity, which could quantificationally detect Hg2+ at the nanomolar level, and the detection limit was 0.49 nM. Moreover, probe R-Hg could be used as a "naked-eye" probe for monitoring Hg2+, because the color of its solution transformed remarkably from yellow into pink with the participation of Hg2+. Importantly, probe R-Hg has potential application value and has been successfully applied to the determination of He2+ in practical water samples and in living RAW 264.7 cells as well as in zebrafish as a bioimaging reagent.