High ultraviolet (UV) stability and low dark current (I-dark) are necessary for high-quality perovskite photodetectors (PDs). TiO2 thin film is known as effective electron-transport-layer (ETL) for perovskite devices. However, common spin-coated TiO2 ETLs endow many surface defects and have strong UV photocatalytic effect to decompose perovskite materials, resulting in inferior stability of devices. In this work, TiO2 bilayer film (Bi-TiO2) has been fabricated by combining spin-coating and atomic-layer-deposition process and its positive effects on UV stability and I-dark of Cs2AgBiBr6-based PDs have been revealed for the first time. It is demonstrated that Bi-TiO2 possesses fewer surface defects and smoother morphology with type II band alignment, which is beneficial to suppress photocatalytic activity of TiO2 and reduce carrier recombination at the interface. After accelerated strong UV aging treatment, the PD with Bi-TiO2 maintains excellent performance, whereas the PD with spin-coated TiO2 film dramatically deteriorate with on-off ratio drops from similar to 102 to similar to 2. Besides, the I-dark of PD remarkably decreases from similar to 10(-8) A to similar to 10(-10) A after bilayer optimization. Furthermore, we have integrated the corresponding PDs into a self-built imaging system adopting diffuse reflection mode. This work suggests a feasible approach to fabricate TiO2/Cs2AgBiBr6-based PDs with remarkable UV tolerance for imaging applications. (C) 2021 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.