A new cyano-substituted poly(p-phenylenevinylene) (PPV) derivative, MEOPS-CNPPV, is synthesized through Knoevenagel condensation of anionic diacetonitrile and neutral dialdehyde and characterized by (1)H NMR, IR, elemental analysis, and gel-permeation chromatography (GPC). To our knowledge, the polymer is the first water-soluble red-emitting PPV derivative. The absorption and emission wavelength of this water-soluble conjugated polymer (CP) depend on the solvent. In buffer solution, the fluorescence of MEOPS-CNPPV is quenched by cationic dinitrobenzene derivatives. Further research indicates that dinitrobenzene derivative with a more flexible structure exhibits a larger K(sv). Making use of the charge reversal of dinitrobenzene-modified substrate, a "turn-on" method is developed for AChE activity assay with the new polymer as a fluorophore. This convenient and direct fluorometric assay thus provides a platform for novel red-emitting sensory systems.