The negative photoresponse characteristics in AlGaN-based p-i-n photodetector have been studied in this paper. The Schottcky contact behavor of p electrode was confirmed to be responsible for this phenomenon. The abnormal photoconductor characteristic was observed in AlGaN photovoltage device under different biases. The persistent photoconductivity characteristic was verified with the capacitance-frequencies experiment under both illumination and dark conditions. The large amount of defects in AlGaN material are considered to be the source of this phenomenon.This paper has systematically studied the abnormal photoresponse behavior and its microscopic mechanism in AlGaN-based p-i-n photodetector, which provides an important foundation for the further optimization of this kind of devices.