To study the sources and erosion process of sediments in the Lhasa River, a systematic analysis was conducted on the grain size, minerals, and geochemical characteristics of the sediments in the main stream of the Lhasa River, as well as its tributaries, Duilonqu River and the Wululongqu River. The results show that: the average particle size of sediments in the Lhasa River is coarser (65.45% sand and 23.64% silt), and the sorting is overall poor; the skewness is mostly positive, and the kurtosis is wide, which reflects the obvious characteristics of river sand sediment. The mineral composition of the Lhasa River sediments is dominated by quartz (38.4%), K-feldspar (27.11%), and plagioclase feldspar (23.64%), followed by clay minerals, and the content of carbonate minerals is relatively low. The illite content in the clay minerals is as high as 83.4%, and the chlorite content is slightly higher than the kaolinite content. Montmorillonite content is very low. The chemical index of illite is less than 0.4, indicating that illite is mainly rich in iron and magnesium. The value of the chemical weathering index (CIA) of the sediments is low, implying that the sediments are in a state of weak and moderate chemical weathering and are dominated by physical weathering. Additionally, the chemical weathering of the sediments in the Lhasa River is not only controlled by the dry and cold climate but also affected by the rapid erosion of extensively exposed granite.