This study presents a year-long PM_2.5 study at Waliguan Baseline Observatory in the northeast of the Tibetan Plateau to investigate the optical properties of water-soluble brown carbon and its source. Our findings highlight that organic matter, sulfate, and nitrate are the dominant contributors to PM_2.5 mass concentrations. Notable seasonal variations in the light absorption capacity of water-soluble brown carbon, accompanied by a high degree of oxidation are also observed.