We have investigated the effect of AlGaN layer parameter on the ultraviolet response of n+-GaN/i-AlxGa1－xN/n+-GaN structure ultraviolet-infrared photodetector and its physical mechanism. Through the simulation, it is found that the decrease of AlGaN background concentration has a positive effect on device’s ultraviolet quantum efficiency. When AlGaN layer background concentration cannot be reduced, the decrease of its thickness can ensure the efficiency. Besides, interfical state should be minimized during materials growth and device fabrication. In addition, small reverse bias voltage can greatly increase ultraviolet quantum efficiency. All these phenomena may be mainly attributed to the existence of the back-to-back heterojunction and the opposite electrical field. It is suggested that we need to adjust structural parameters to obtain high quantum efficiency according to the materials quality in device design.