A tapered thin-diameter photonic crystal fiber interferometer (TTD-PCFI) biosensor is proposed and demonstrated for DNA hybridization detection with an ultra-low limit of detection (LoD) and high specificity. The TTD-PCFI is fabricated by arc-discharged melting and adiabatic tapering, with a lower loss and stronger evanescent optical field. The fabricated TTD-PCFI with a diameter of 8.63 µm achieves a high refractive index sensitivity of 3842.5 nm/RIU. DNA hybridization detection of concentration ranging from 1 fM to 1 pM is achieved, with a sensitivity of 632 pm/lg(fM). The measured LoD is as low as 410 aM. Single-base mismatch detection of 25-base target single-stranded DNA is also achieved. The proposed label-free DNA biosensor has the advantages of the ultra-low LoD, high specificity, and real-time detection, which has great application prospects in the fields of disease diagnosis, microbial detection, and environmental science.