The invention relates to the technical field of fluorescent sensing material preparation and biodetection, and particularly relates to a fluorescent probe simultaneously detecting ONOO<-> and H2S, anda synthesis method and an application thereof. The probe molecule has two reaction sites, one can be subjected to nucleophilic addition reaction with H2S, excitation wavelength being 480 nm and emitting wavelength being 580 nm, thus significantly enhancing a fluorescent signal; the other can be subjected to nucleophilic reaction with ONOO<->, excitation wavelength being 400 nm and emitting wavelength being 520 nm, then a rearrangement reaction is carried out to obtain two coumarin derivative products, the fluorescent signal, after the reactions, being significantly enhanced too. Therefore, RC-Br can detect the ONOO<-> and H2S in a double-excitation/double-emission manner. The fluorescent probe has high sensitivity and responding speed, can be applied to cell imaging well, and may be an effective analytic tool for researching interaction relationship of the ONOO<-> and H2S in a biology system.