Accurate analysis of circular RNA (circRNA) is essential for the elucidation of circRNA-mediated signaling pathways and its association with diseases. Here we present a new tool, namely, linear DNA nanostructure (LDN), for efficient assay of circRNA. In this assay, target circRNA initiates cascade displacement reaction between two hairpin probes sequentially assembled along the LDN and finally lights up the whole LDN. Meanwhile, the target circRNA can be recycled and serve as a catalyst for multiple LDNs lighting, leading to the signal switch from "OFF" to "ON". Systematic studies reveal that our LDN-based method can strictly recognize and rapid respond to circRNA with high signal gain. Furthermore, by using human tumor cells it has been verified that the LDN-based method can also be used for easy and high-sensitive imaging of intracellular circRNA, which provides a useful platform for assaying low-abundance circRNA in cells.