Self-powered Ultraviolet (UV) photodetectors have attracted great attention for the fast detection of UV light without external bias voltage, as well as for their high sensitivity, light weight and low energy consumption. In order to obtain high-performance self-powered UV photodetector, Bi₂O₃ nanoblocks and g-C₃N₄ nanosheets were synthesized by thermal polymerization method. Subsequently, Bi₂O₃/g-C₃N₄ composites were prepared via solution method, and their morphology, crystal structure, chemical composition and valance state were characterized. The results of morphological characterization show that Bi₂O₃ present as a honeycomb-structured block, which are attached to the surface of g-C₃N₄ nanosheets with layered structure. The characteristic peaks of Bi₂O₃ nanoblocks and g-C₃N₄ nanosheets can be observed in the XRD pattern of the as-prepared composite, indicating successfully constructing of the Bi₂O₃/g-C₃N₄ heterojunction. Further, XPS measurement of the Bi₂O₃/g-C₃N₄ composite was performed to study the valence states of the elements compositing the composite, in which the results of high-resolution XPS of Bi 4f, O 1s, C 1s and N 1s also confirm the prepared composite as Bi₂O₃/g-C₃N₄ heterojunction. Based on the as-prepared Bi₂O₃/g-C₃N₄ heterojunction, the UV photodetector that can operate without an external power source was fabricated. At zero bias voltage, the photodetection performance of the Bi₂O₃/g-C₃N₄ UV photodetector was investigated under darkness and UV illumination by repeating 10 s with light on and 10 s with light off. When irradiated with UV light, the Bi₂O₃/g-C₃N₄ UV photodetector immediately generates photocurrent and rapidly rises to the maximum value (0.43 μA) and remain stable. After turning off the UV light, the photocurrent immediately decreases and returns to the initial state. Notably, the maximum photocurrent of the device remains essentially unchanged after 7 cycles, indicating that the Bi₂O₃/g-C₃N₄ UV photodetector has good stability and repeatability. In addition, the performance of the UV photodetector prepared based on Bi₂O₃ nanoblocks was also investigated under the same conditions. The results show that when the Bi₂O₃ nanoblocks UV photodetector was exposed to UV light, its photocurrent can rise rapidly to reach the maximum value of about 0.21 μA. Remarkably, the maximum photocurrent of the Bi₂O₃/g-C₃N₄ UV photodetector is enhanced by about 1.05 times compared with that of the Bi₂O₃ nanoblocks UV photodetector. And the Bi₂O₃/g-C₃N₄ UV photodetector exhibits a fast response time (~181.7 ms), which demonstrates that the device is capable of stable and fast detection of UV light. The photoresponse characteristics of the Bi₂O₃/g-C₃N₄ UV photodetector were also investigated under UV light irradiation with different intensities. It is found that even under UV irradiation at a lower light intensity (2 mW/cm²), the photocurrent generated by the device increases rapidly as well as still reaches a maximum value of about 0.098 μA. And there is no significant attenuation of the maximum photocurrent after several cycles of cycling. Meanwhile, note that the photocurrent increases approximately linearly with the increase of the light intensity, which indicates that there has a well linear relationship between the photocurrent and incident light intensity. And the device also exhibits excellent recyclability and stability for the detection of UV light of different intensities, which demonstrates the ability of the device to achieve effective detection of UV light over a wide range of UV intensities. The device also exhibits a large switching ratio with a value of about 1 313, which demonstrates the high sensitivity of the Bi₂O₃/g-C₃N₄ UV photodetector. And its responsivity is as high as 0.17 mA/W under UV irradiation of 2 mW/cm², demonstrating the potential application of the device in the field of photodetectors. In addition, the detection mechanism of the Bi₂O₃/g-C₃N₄ UV photodetector is analyzed. The high-performance Bi₂O₃/g-C₃N₄ self-powered UV photodetector prepared in this work may be used in the field of future optoelectronic devices.