Interface engineering is an effective way to improve the performance of graphene-based photodetectors. Here, a graphene/GaAs heterojunction photodetector is fabricated, and when inserting an Al₂O₃ tunneling layer, its performance is improved through direct tunneling (DT) and Fowler-Nordheim tunneling (FNT). According to the experimental results, it is found that the thickness of the tunneling layer has a great influence on the performance of the photodetector. Compared with graphene/GaAs photodetector, the performance of graphene/Al₂O₃(2 nm)/GaAs photodetector is significantly improved with the responsivity, detectivity, and external quantum efficiency value of 0.80 A/W, 3.02 × 10¹¹ Jones and 306% under 1 mW/cm² light intensity at 2 V bias. Meanwhile, fast response is also observed (rise/decay time of 3 ms/8.6 ms). The improvement of the photodetector's performance in this work is mainly attributed to the effective modification of the interface state by the Al₂O₃ tunneling layer and the effect of the two tunneling mechanisms based on DT and FNT.