Reductive cleavage of alkenes is rarely reported in synthetic chemistry. Here we report a unique H2S-mediated reductive cleavage of C=C under mild condition, which is approved to be a new strategy for the design of probes for effective sensing H2S with turn-on dual-color fluorescence. A short series of phenothiazine ethylidene malononitrile derivatives were proved to react with H2S, via reductive cleavage of C=C with intramolecular cyclization reaction to form thiophene ring. Enlightened by this new reaction mechanism, four effective probes with turn-off to turn-on fluorescence switch were successfully applied for sensing H2S, an important gaseous signaling molecule in living body, among which PTZ-P4 exhibited two fluorescent colors after reductive cleavage. The dual-color probe was applied to image endogenous H2S and showed distinct difference in brightness in living C. elegans for wild type N2, glp-1 (e2144) mutants (higher level of endogenous H2S), and cth-1 (ok3319) mutants (lower level of endogenous H2S). The discovery of H2S-mediated reductive cleavage of C=C is expected to be valuable for chemical synthesis, theoretical study, and design of new fluorescent H2S probes.