Bismuth-based perovskites are considered to be promising candidates to substitute the toxic lead-based perovskite in optoelectronics due to their excellent optoelectronic properties, high environmental friendliness, and (moisture, light, and heat) stability. However, there are still few reports about high performance bismuth-based perovskite ultraviolet photodetectors, and is more lacking in ultraviolet imaging demonstration. Herein, we reported a self-powered NiO_x/Cs₃Bi₂Br₉ heterojunction photodetector with excellent photodetection performance by electrochemical depositing NiO_x as the hole transport layer. The optimized NiO_x/Cs₃Bi₂Br₉ heterojunction photodetector exhibits excellent ultraviolet detection performance with a fast response speed of 3.04/4.65 ms, wide linear dynamic range of 116.6 dB, decent responsivity of 4.33 mA·W⁻¹ at 0 V bias, and high detectivity of 1.3 × 10¹¹ jones. The outstanding performance of the optimized NiO_x/Cs₃Bi₂Br₉ heterojunction photodetector is enough to meet the high-quality ultraviolet imaging. Therefore, we further integrated the optimized NiO_x/Cs₃Bi₂Br₉ heterojunction photodetector to the transmission mode ultraviolet multispectral imaging system, achieving admirable imaging results at weak light condition. This work will play a positive role in promoting the development of bismuth-based ultraviolet photodetection and ultraviolet multispectral imaging.