Reductive cleavage of alkenes is rarely reported in synthetic chemistry. Here we report a unique HS-mediated reductive cleavage of C═C bonds under mild conditions, which is a successful new strategy for the design of probes for effective sensing of HS with turn-on dual-color fluorescence. A short series of phenothiazine ethylidene malononitrile derivatives were shown to react with HS, via reductive cleavage of C═C bonds with intramolecular cyclization reactions to form thiophene rings. Enlightened by this new reaction mechanism, four effective probes with turn-off to turn-on fluorescence switches were successfully applied for sensing HS, an important gaseous signalling molecule in living systems, among which PTZ-P4 exhibited two fluorescent colors after reductive cleavage. The dual-color probe was applied for imaging endogenous HS and showed distinct differences in brightness in living C. elegans for wild type N2, glp-1 (e2144) mutants (higher levels of endogenous HS), and cth-1 (ok3319) mutants (lower levels of endogenous HS). The discovery of HS-mediated reductive cleavage of C═C bonds is expected to be valuable for chemical synthesis, theoretical studies, and the design of new fluorescent HS probes.