Abstract: In order to explore the impact of mining activities on river water quality, the authors chose the Dongchuan River basin, downstream of a large molybdenum ore area in the Qinling Mountains, as the research object. Through on-site sampling and indoor testing, the characteristics of heavy metal Cd in surface water, sediments, and rock samples were analyzed. The contamination status and ecological risk assessment of the river and sediment samples in the Dongchuan River basin were evaluated using the exceeding pollution standard multiple method, the geoaccumulation index method, and the Hakanson potential ecological risk assessment method, with a discussion on the source of Cd. The results show that the Cd content of the 16 river sampling sites in the main channel of Dongchuan River was significantly higher than that in the control sites that are not affected by mining activities, the coefficient of variation of Cd content reached 95.65% and the Cd content from upstream to downstream showed a skewed distribution; the information shows that Cd in the Dongchuan River basin was significantly affected by mining activity. The Cd content of 93.75% of the 16 river water sampling points in the main channel of Dongchuan River meets class Ⅱ water standard for surface water, and it can be judged that the overall water quality of Dongchuan River is better. Only the No. 5 point exceeds the class Ⅱ water quality standard. The multiple of exceeding pollution standard is 1, suggesting light pollution. The results of geoaccumulation index analysis show that 11 sediment samples from the main channel of Dongchuan River had different levels of pollution:sampling point 10, the most seriously polluted point, reached the moderate-strong pollution level; sampling points 8, 9 and 13, the least polluted point, were pollution-free and moderately polluted; and most sites were moderately polluted. The average value of the potential ecological risk index of heavy metal Cd in 11 samples of Dongchuan River's main canal was 120.27, indicating that the total sediments of the main channel of Dongchuan River reached a level of strong ecological risk. Acid mine drainage, primary geological background, sediment release, atmospheric dry and wet dust fall seem to be the major pollution sources of heavy metal Cd in the Dongchuan River basin.