Flexible devices provide advantages such as conformability, portability, and low cost. Paper-based electronics offers a number of advantages for many applications. It is lightweight, inexpensive, and biodegradable, making it an ideal choice for disposable electronics. In this work, we propose a novel configuration of photodetectors using paper as flexible substrates and amorphous ${\mathrm{Ga}}_2{\mathrm{O}}_3$ as the active materials, respectively. The photoresponse characteristics are investigated systematically. A decent responsivity yield and a specific detectivity of up to 66 mA/W and $3\times {10}^{12}$ Jones were obtained at a low operating voltage of 10 V. The experiments also demonstrate that neither the twisting nor bending deformation can bring obvious performance degradation to the device. This work presents a candidate strategy for the application of conventional paper substrates to low-cost flexible solar-blind photodetectors, showing the potential of being integrated with other materials to create interactive flexible circuits.