The capability of in situ detection of microRNA in living cells with signal amplification strategy is of fundamental importance, and it will open up a new opportunity in development of diagnosis and prognosis of many diseases. Herein we report a swing DNA nanomachine for intracellular microRNA detection. The surfaces of Au nanoparticles (NPs) are modified by two hairpin DNA. We observe that one DNA (MB2) will open its hairpin structure upon partial hybridization with target miR-21 after entering into cells, and the other part of its hairpin structure could further react with the other hairpin DNA (MB1) to form a Zn-specific DNAzyme. This results in the disruption of MB1 through shearing action and the release of fluorescein Cy5. To provide an intelligent DNA nanomachine, MB2 is available again with the shearing action to bind with MB1, which provides effective signal amplification. This target-responsive, DNA nanomachine-based method showed a detection limit of 0.1 nM in vitro, and this approach could be an important step toward intracellular amplified detection and imaging of various analytes in living cells.