The salinization of freshwater resources constitutes an increasingly global challenge, exacerbated by climate change and human activities. Despite its growing significance, comprehensive assessments of salinity dynamics and the roles of natural and anthropogenic factors remain scarce. This study investigates surface water salinity levels and their long-term (2003-2022) and seasonal trends across 1356 sampling sites in ten major Chinese river basins. Our results reveal that >20 % of the sites exhibit relatively high long-term average salinity levels compared to the irrigation water threshold, primary in arid or semi-arid regions experiencing intensified human activities. Nearly 20 % of low-salinity sites exhibit significant trends towards increased salinity, primarily in humid region. Southern basins, such as the Pearl River and Yangtze River, generally have relatively low salinity but demonstrate upward trends, whereas northern basins, like the Yellow River and Huai River, experience moderate to high salinity levels with more rapid increases. Winter salinity levels and their rate of increase surpass those of other seasons. Anthropogenic drivers, particularly population density and agricultural water use, emerge as key contributors to rising salinity, in conjunction with hydroclimatic variables. Furthermore, seasonal salinity trends underscore the critical role of agricultural water use during summer and autumn months. These findings emphasize the necessity to address the compounded pressures of climate variability and human activities, which are increasingly threatening surface water quality through rising salinity and extreme weather events.