In this work, we design and synthesize a new near-infrared (NIR) ratiometric fluorescent probe FD-H₂S for the highly sensitive (DL 68.2 nM) detection of H₂S with fast response (15 s), large emission shift (220 nm) and excellent enhancement (168-fold in ratiometric value). The probe could be applied for monitoring and imaging of exogenous or endogenous H₂S in live MCF-7 cells and in live mice with the fastest response.