A practical method for organic–inorganic hybrid heterojunction photodetector by spin-coating the cobalt phthalocyanine (CoPc) solution onto the β-gallium oxide (β-Gasub.2Osub.3) film is available to realize the sensitive detection of ultraviolet light signals. We have carefully measured and calculated the optoelectronic performance of the prepared device. The device demonstrates excellent rectification characteristics under different light intensities, and the rectification ratio reaches 144.96 under 900 μW/cmsup.2 at ±5 V. Benefiting from the construction of CoPc/Gasub.2Osub.3 heterojunction, the device shows an extremely low dark current of 5.73 fA, a high detectivity of 1.92 × 10sup.17 Jones, a responsivity of 18.4 mA/W, and a high light-to-dark current ratio of 3.76 × 10sup.6. In addition, the intrinsic physical mechanism of the device is investigated through the energy band diagram under different conditions. The device is equipped with the possibility to work under self-powered mode and has good stability in the air environment.