Enzyme-free DNA strand displacement process is often practical when detecting miRNAs expressed at low levels in living cells. However, the poor kinetics, tedious reaction period, and multicomponent system hamper its in vivo applications to a great extent. Herein, we design a branch-shaped trapping device (BTD)-based spatial confinement reactor and applied it for accelerated miRNA in situ imaging. The reactor consists of a pair of trapped probe-based catalyzed hairpin assembly (T-CHA) reactions attached around the BTD. The trapping device naturally offered CHA reactions a good spatial-confinement effect by integrating the metastable probes (MHPa and MHPb) of the traditional CHA with the four-branched arm of BTD, which greatly improved the localized concentration of probes and shortened their physical distance. The autonomous and progressive walk of miRNA on the four-arm nanoprobes via T-CHA can rapidly tie numerous four-arm nanoprobes into figure-of-eight nanoknots (FENs), yielding strong fluorescence that is proportional to the miRNA expression level. The unique nanoarchitecture of the FEN also benefits the restricted freedom of movement (FOM) in a confined cellular environment, which makes the system ideally suitable for in situ imaging of intracellular miRNAs. In vitro and in situ analyses also demonstrated that the T-CHA overall outperformed the dissociative probe-based CHA (D-CHA) in stability, reaction speed, and amplification sensitivity. The final application of the T-CHA-based four-arm nanoprobe for imagings of both cancer cells and normal cells shows the potential of the platform for accurately and timely revealing miRNA in biological systems.