Accurate and specific imaging of low-abundance intracellular microRNAs (miRNAs) is crucial for monitoring cellular processes and disease diagnosis. Despite the fact that nucleic acid amplification technologies have shown great advantages for the detection of trace targets, their live cell imaging applications remain a major challenge because of the insufficient stability and slow reaction kinetics of the probes in cellular delivery and imaging. Herein, we demonstrate the synthesis of DNA-cross-linked polymeric lighting-up nanogels (DPLNs) through the DNA hairpin-based hybridization chain reaction within nanoscale-confined space for monitoring and imaging live cell miRNA-21 with high sensitivity. Cascaded catalytic hairpin assembly of two hairpin signal probes confined in the DPLNs can be triggered by the target miRNA, causing substantially amplified fluorescence resonance energy transfer signals with accelerated reaction kinetics. Moreover, the DPLNs show low cytotoxicity and highly enhanced nuclease resistance and can be successfully delivered into live cells for imaging low levels of miRNA-21. In addition, the DPLN probes can be readily tuned by specific sequences for monitoring various molecular targets in live cells for important biological and biomedical applications.