The three-dimensional (3D) DNA nanostructure has been got much attention due to its excellent biocompatibility, enhanced structural stability, highly programmable and perfect cell-delivery performance. Here, a novel 3D DNA tetrahedron amplifier (DTA) has been developed for rapid and efficient mRNA imaging in living cells using target catalyzing spatial-confinement hairpin DNA assembly cascade reaction inside the DNA nanostructure. The DTA was constructed by assembling a DNA tetrahedron with four DNA strands at first, and then by assembling two metastable DNA hairpins H1 (Cy5) and H2 (Cy3) at specific locations of the DNA tetrahedron. In the presence of target mRNA, the catalyzed hairpin assembly (CHA) reaction on the DTA could be triggered and a H1-H2 duplexes nanostructure could be formed, which would obtain a significant fluorescence resonance energy transfer (FRET) signal, and release the target mRNA could trigger next H1-H2 duplexes formation. Due to the 3D DNA tetrahedral spatial-confinement effect, the circular reaction of DTA could achieve rapid and efficient amplification detection of target mRNA in living cells. Moreover, the DTA show excellent structural stability and non-cytotoxicity. This strategy presents a versatile method for the ultrasensitive detection of biomarkers in living system and gains a deeper development of the DNA nanostructures in biomedical functions.