A new method to reduce the dark current of GaN based Schottky barrier ultraviolet photodetector is proposed. In comparision with conventional i-GaN/n＋-GaN structure, an additional thin p-GaN cap layer is introduced on the i-GaN(n--GaN) in the new structure. The simulation results showed that the additional layer makes the dark current to decrease in the photodetector due to the increase of the Schottky barrier height. The effects of thickness and carrier concentration of p-GaN layer on the dark current of the photodetector were also studied. It is suggested that the dark current of the new structure device could be better reduced by employing p-GaN with higher carrier concentration as the cap layer.