A new quinoline-based fluorescent chemosensor MDCQA was designed and synthesized for the relay sensing of Zn2+ and S2-. MDCQA showed turn-on fluorescence toward Zn2+, and the resulting complex was able to detect S2- among the competitive anions by turn-off fluorescence. Detecting limits (0.06 mu M and 2.33 mu M) for zinc ion and sulfide were remarkably below WHO guidelines (76.0 mu M for zinc ion and 14.7 mu M for sulfide). In addition, sensor MDCQA successfully operated for imaging Zn2+ in live zebrafish larvae and quantifying it in real water samples. The detecting properties and mechanisms of MDCQA toward zinc ion and sulfide were demonstrated by UV-vis and fluorescence spectroscopy, NMR titration, DFT calculations and ESI-Mass.