Lutetium oxide (Lu₂O₃) has an ultra-wide bandgap of 5.5–6.2 eV and great potential for application in high-performance deep ultraviolet photodetectors (PDs). In this work, a strategy to improve the performance of Lu₂O₃-based DUV PDs is proposed, which makes use of the photo-induced pyro-phototropic effect and the photovoltaic effect (that is, the dual-field coupling effect) of the heterojunction. By constructing a Ti₃C₂T_x/Lu₂O₃ Schottky heterojunction and utilizing the photo-dual-field coupling effect induced at the heterojunction interface, an endogenous synergistic enhancement is realized in the efficiency of DUV optical absorption and photogenerated carrier separation. The Ti₃C₂T_x/Lu₂O₃/GaN DUV photodetector fabricated in this study exhibits excellent performance at 0 V bias, such as a high photoresponsivity (R ≈ 23.8 mA W⁻¹) and a high response speed (decay time ∼40.3 ms). Under the same illumination conditions, the photoresponsivity of this device based on the dual-field coupling effect is higher than that based on the single photovoltaic effect. What's more, this device also has better DUV photoelectric properties when compared with Ag/Lu₂O₃/GaN detectors. These results indicate that the dual-field coupling effect induced by light in the Ti₃C₂T_x/Lu₂O₃ heterojunction is key to enhancing the detection performance of Lu₂O₃-based DUV PDs. This work provides new ideas for the application of Lu₂O₃ wide-bandgap semiconductors in high-performance DUV photodetectors.