The spectral response range of the high Al-content AlGaN material is in the solar-blind ultraviolet band, but the higher trap density on its surface limits its application on the fields of solar-blind ultraviolet detection. Therefore, the effect of surface modification on the performance of AlGaN-based solar-blind ultraviolet photodetectors was investigated. The chemical self-assembly method was adopted to chemically adsorb octadecanethiol organic molecules on the surface of the Al_0.6Ga_0.4N wafer to prepare a surface-modified AlGaN-based photodetectors. Compared with unmodified photodetector, this modification process could effectively reduce the adverse effects caused by the surface states of the AlGaN material, reduce the leakage dark current, and significantly improve the responsivity of the device, indicating the realization of AlGaN-based metal-semiconductor-metal solar-blind photodetectors with high performance.