The trends of water quality change in the Yangtze River and the Yellow River in last 40 years were studied based on monitoring data collected from the Water Year Book of China (from the late 195 to the middle 1990´s) and the UN GEMS/Water program (1979〜1994). The results show that these two rivers each had their own trend of water quality change in respect of the major ions content. The water acidification trend mainly occured in the upper and middle reaches of the Yangtze River, which was caused by serious acid deposition due to the increasing S0₂ emission from coal combustion oxidation of the increasing nitrogenous fertilizer runs from farmland. The water quality concentration trend happened in the Yellow River, which was related to the decreasing of run off and the increasing of irrigation water. The total ion content of river water was increased by the concentration of irrigation water in farmland and its washing effect on soils.