High polymerization degree is critical for polypyrene applying in fluorescent sensing field, which closely related to high sensitivity. However, polypyrene films reported are either insoluble and unprocessable or soluble with few repeat units. In this work, we synthesized a poly(1-pyrenebutyric acid) (PPBA) via electrochemical polymerization in boron fluoride ethyl ether system and the degree of polymerization was up to 43, which showed good fluorescence. The polymer showed high selectivity toward to Cr2O72− among sixteen anions and twenty cations in ethyl alcohol system, and the limit of detection was as low as 1.58 fM. Transmission electron microscope results showed that the quenching mechanism of the PPBA was Cr2O72−-induced inter- and intra-polymer aggregation of polymer chains in solution. Finally, PPBA was successfully applied in milk detection, implying that PPBA can realize the ultra-trace detection of Cr2O72− in practical application.