Based on the groundwater level measured in 2009 in the downstream of Yellow River plain, combined with the isotopic (¹⁸O and ³H)geochemistry of surface water and the groundwater, the lateral seepage scope and variation of downstream of Yellow River were analyzed after 20 years' operation of Xiaolangdi reservoir. The result indicates that in 20 years of operation of Xiaolangdi reservoir, the current scope of lateral seepage of the downstream of Yellow River will be between 5-25 km, with the largest scope of about 25 km in Xinxiang-Kaifeng and the smallest scope of 5 km in Binzhou. Compared with affected scope of lateral seepage in the initial operation of Xiaolangdi reservoir, the affected scope increased by about 0.5-5 km in the west of Binzhou, with most significant increment along Xinxiang-Zhengzhou, and slight decrement in the east of Binzhou. The affected scope of lateral seepage was mainly controlled by medium condition, recharge source condition and energy condition. After Xiaolangdi reservoir was put into operation, the channel was cut down, the permeability of the river bed was enhanced, and the "adsorption and purification" ability of the river bed material to the Yellow River water was reduced. Then, the water quality of Yellow River has a significant impact on the groundwater quality along the river. Hence, the control and improvement of water quality of Yellow River was the basis to ensure the high-quality development of the ecological environment in the basin of down-Yellow River.