Thioflavin T (ThT) has been widely utilized as a fluorescent marker for amyloid fibrils. However, the use of ThT as an efficient reporter for a specific DNA structure still remains in question. Here, we report that the fluorescence intensity of ThT is obviously enhancement in when it binds to ds-DNAs which contain cavity structures such as an abasic site, gap site or mismatch site. Such enhancement in fluorescence cannot be achieved for DNA without these cavity structures. The DNA cavities provide appropriate spaces to accommodate ThT and allow the occurrence of some specific interactions. The stacking interaction of the bound ThT with the cavity context bases is the main driving force for ThT binding to the cavities. This interaction restricts the excited state's rapid torsional rotation around the single C-C bond between the benzothiazole and dimethylaminobenzene moieties and thus results in a decreased population of the nonradiative twisted internal charge-transfer (TICT) state. It is impossible for this stacking interaction to occur in DNA without these cavities. This property can be used to recognize DNA cavities with high selectivity and sensitivity. We expect that the ability of ThT to target these DNA structures has the potential to be developed into practical and functional biomaterials for DNA sensors or devices.