In this work, a highly sensitive impedimetric biosensor was developed for mercuric ion (Hg2+) detection. The biosensor design was based on Hg2+-triggered exonuclease III (Exo III) cleavage for target recycling and DNAzyme-mediated catalytic for precipitation polymerization. Hg2+ induced thymine-thymine (T-T) mismatches were used to trigger the Exo III-catalyzed target recycling and produce free single-stranded DNA (defined as M). The outputted M then assisted the in formation of a DNA network on electrode surface to efficiently immobilize the porphyrin manganese (MnTmPyP). The formed MnTMPyP-double-stranded DNA (MnTmPyP-dsDNA) complex exhibited peroxidase-like activity capable of catalyzing a 3,3-diaminobenzidine (DAB) oxidation reaction, which produced an insoluble precipitate on the electrode surface. This reaction significantly enhanced the resistance signal for the quantitative determination of Hg2+. Under optimal conditions, the impedimetric biosensor exhibited a wide dynamic working range of 0.005 nM–100 nM with a detection limit of 1.47 pM. This platform also demonstrated good reproducibility and selectivity, offering a promising avenue for the detection of other molecules. Display omitted •T-Hg2+-T mismatches and Exo III are adopted for target converting and recycling.•MnTMPyP-dsDNA peroxidase-like DNAzyme is used for precipitation polymerization.•Biosensor shows good performances and is applied to Hg2+ detection in real samples.