A new ratiometric fluorescent cyanide probe bearing a benzo [e] indolium moiety as a fluorophore and binding site was developed. The detection of cyanide was performed via the nucleophilic attack of cyanide toward the benzo [e] indolium group of the probe, resulting in a prominent fluorescence ratiometric change and a color change. The probe was invested in water with high selectivity and sensitivity for cyanide. A simple paper test strip system for the rapid monitoring of cyanide was developed.