Two poly(pyrene-co-phenyleneethynylene)s of different compositions (PyPE-1 and PyPE-2) were synthesized and characterized. The two polymers had been casted, separately, onto glass plate surfaces to fabricate films (film 1, film 2) for sensing performance studies. It has been demonstrated that the fluorescence emissions of the two films are sensitive to the presence of 2,4,6-trinitrotoluene (TNT) in aqueous phase. Interestingly, TNT shows little effect upon the emission of the parent polymer, poly(phenyleneethynylene) (PPE). The difference was explained by considering (1) the π–π interaction between pyrene moieties contained in the copolymers and the analyte, TNT, molecules, and (2) more suitable matching of the LUMOs (lowest unoccupied molecular orbital) of the pyrene-containing conjugated polymers with that of TNT molecules. Further experiments demonstrated that the sensing is reversible and rarely encounters interference from commonly found compounds, including other nitroaromatics (NACs). Fluorescence lifetime measurements revealed that the quenching is static in nature. The smart performance of the films and the easiness of their preparation guarantee that the films may be developed into sensor devices for the supersensitive detection of TNT in groundwater or seawater.