A new GaN-based ultraviolet photodetector with Schottky barrior structure is proposed. Comparied with the conventional i-GaN/n+-GaN structure，there is an additional thin n-AlGaN cap layer on the i-GaN in the new structure. The simulation result demonstrates that the new structure leads to an increased quantum efficiency in GaN photodetection，since the negative effect of surface states on the photodetector is reduced in the new structure. In addition，it is suggested that the performance of device with the new structure could be further improved by employing an even thinner AlGaN cap layer with higher carrier concentration.