A new ultraviolet photodetector of employing p menus type GaN （p－-GaN） as the active layer is proposed. It is easy to obtain the p－-GaN layer with low carrier concentration. As a result, the depletion region can be increased and the quantum efficiency can be improved. The influence of some structure parameters on the performance of the new device is investigated. Through the simulation calculation, it is found that the quantum efficiency increases with the decrease of the barrier height between the metal electrode and the p－-GaN layer, and it is also found that the quantum efficiency can be improved by reducing the thickness of the p－-GaN layer. To fabricate the new photodetector with high performance, we should employ thin p－-GaN layer as the active layer and reduce the Schottky barrier height.