The poly(9,9-dioctylfuorenyl-2,7-diyl)-alt-co-(1,4-benzo-{2,1′,3}-thiadiazole) (PFBT) was carboxyl-functionalized to prepare polymer dots (C-PFBT Pdots), which served as a self-ECL emitter for producing an extraordinary ECL signal without any exogenous coreactants. The C-PFBT Pdots–modified electrode captured the substrate DNA and further hybridized with a ferrocene (Fc)-labeled DNA. The ECL emission of C-PFBT Pdots was quenched by Fc (a signal off state). After the DNAzyme was added, the DNAzyme-substrate hybrids were formed through hybridizing between DNAzyme and substrate and the Fc-labeled DNA was released. In the presence of target Pb 2+ , the DNAzyme-substrate hybrids could be specifically recognized and cleaved to release the DNAzyme and Pb 2+ . Ultimately, the released DNAzyme would further hybridize with the substrate for producing the DNAzyme-substrate hybrids and then were cleaved by the released Pb 2+ . As a result, the DNA walking machine was generated and the substantial Fc was away from C-PFBT Pdots to obtain a signal on state. Such a strategy achieved a sensitive detection of Pb 2+ and the detection limit was as low as 0.17 pM. Moreover, making this ECL biosensor for an intracellular Pb 2+ detecting, a convincing performance was achieved. The self-ECL emitter C-PFBT Pdots combining with the quencher Fc provided a new strategy and platform for constructing a coreactant-free ECL assay.