Abstract Ambient aerosols exert growing influences on atmospheric radiative forcing, precipitation, and climate change over the Tibetan Plateau (TP), yet observational constraints on their chemical characteristics remain sparse in this high‐altitude region. A 2‐month field study was conducted during the pre‐monsoon season at Yarlung Tsangpo Grand Canyon, the primary water vapor channel into the TP, to investigate aerosol sources and chemical evolution in this critical corridor. The campaign‐averaged mass concentration of submicron aerosols was 5.68 μg m−3, dominated by secondary species (78.2%). Notably, a large contribution (27.8%) of aqueous‐phase‐processed oxygenated organic aerosol (aq‐OOA) was observed in the early pre‐monsoon period, closely linked to elevated aerosol liquid water (ALW) and transboundary transported biomass‐burning emissions. Our results highlight the importance of ALW‐involved aqueous‐phase processing of secondary aerosols associated with South Asian outflow in this moisture‐rich channel, which is crucial for evaluating their interactions with clouds and impacts on precipitation in the southeastern TP.