DNAzymes have been recognized as promising transducing agents for visualizing endogenous biomarkers, but their inefficient intracellular delivery and limited amplification capacity (including insufficient cofactor supply) preclude their extensive biological application. Herein, an autocatalytic DNAzyme (ACD) biocircuit is constructed for amplified microRNA imaging in vivo based on a hybridization chain reaction (HCR) and DNAzyme biocatalysis, sustained by a honeycomb MnO2 nanosponge (hMNS). The hMNS not only delivers DNA probes, but also supplies Mn2+ as a DNAzyme cofactor and magnetic resonance imaging (MRI) agent. Through the subsequent cross‐activation of HCR and DNAzyme amplicons, the ACD amplifies the limited signal resulting from miRNA recognition. The hMNS/ACD system was used to image microRNA in vivo, thus demonstrating its great promise in cancer diagnosis. An autocatalytic circuit is constructed for in vivo fluorescence/molecular resonance imaging based on a hybridization chain reaction (HCR) and DNAzyme biocatalysis, sustained by a honeycomb MnO2 nanosponge (hMNS). The hMNS was not only used to deliver the DNA, but also to supply the DNAzyme with Mn2+ as a cofactor. This system was used to detect microRNA in vivo and shows great promise in cancer diagnosis.