The security of the environment in the headwaters is vital to the whole basin. This study focused on the main ion composition, spatial distribution characteristics, and sources of river water and surface sediments in the source regions of the Lancang River and the Yangtze River and analyzed hydrochemical type and control factors. The results showed that the main cations in the water of the source region were Ca²⁺ and Na⁺, with a combined proportion of over 89%. The main anions were Cl^-, SO₄^2-, and HCO₃^-, with a combined proportion of over 97%. There were some differences in the content of metallic elements in the water. The average contents of As, Cu, Pb, Cr, Cd, Ni, and Ti in the river water of the source region of the Yangtze River (SRYR) were higher than those in the source region of the Lancang River (SRLR). The main hydrochemical types in the source region were Na-Cl and Ca-SO₄•HCO₃, which were controlled by rock weathering and evaporating crystallization. The ions in the river water of the source region were less affected by atmospheric transport and human activities, but more affected by rock weathering. In the SRLR, the water ions were mainly affected by evaporites and carbonate rocks. And in the SRYR, those were mainly affected by evaporites, silicates, and carbonate rocks. The average contents of Ti, Mn, Fe, Sr, and Ba in the surface sediments of rivers were higher than those of other metallic elements. The metallic elements in sediments were controlled by natural and traffic pollution. The study could provide basic data and a scientific basis for the study of the geochemical processes of the water environment and the protection of river ecological environments in plateau rivers.