This work describes the design and TCAD-based simulation of a high-speed self-powered ultraviolet (UV) photodetector based on a heavily p-type doped multi-layer graphene (p⁺-MLG) and lightly n-type doped ZnO nanowires (n⁻-ZnO NWs) heterojunction. The device structure is based on the utilization of the high built-in electric field across the heterointerface for effective carrier separation to support both self-biasing and photoconductive operations. Simulated outcomes show rectifying characteristics with a dark-mode and light-mode rectification ratios of 8.5 × 10³ and 5.2 × 10², respectively. Spectral response demonstrates a maximum photocurrent-to-dark current ratio at 350 nm. The device has an external quantum efficiency (QE) of 55.43%, maximum responsivity (R_i) of 0.16 A/W, detectivity (D^*) of 2.44 × 10⁹ Jones, and rapid photoswitching times with both rise time and fall time of 0.16 ns. Compared to previously reported graphene/ZnO photodetectors, the proposed heterostructure demonstrates enhanced photoswitching speed and efficient self-powered operation due to the optimized heterojunction design and strong internal electric field. p⁺-MLG/n⁻-ZnO NWs integration facilitates UV detection with mechanical flexibility and low power consumption, which makes the device of great interest for wearable and biomedical sensing devices.