As the major control engineering on the main stream of the Yangtze River, the impoundment of the Three Gorges Project has resulted in significant hydrological changes of the downstream. Based on the data of water level and flow in July and August collected from three hydrometric stations on the Yangtze River and the Dongting Lake (Jianli, Chenglingji, and Luoshan Stations) during 1990 and 2017, this paper employs methods of correlation analysis, contrastive analysis and discharge ratio to study the effect of operation of the Three Gorges Reservoir in July and August on the hydrology of confluence section of the downstream rivers and lakes. The results showed that: the percentage of outflow discharge account for the total inflow discharge at Jianli Station increased from 58.81% to 60.53%, whereas that of the Chenglingji Station decreased from 43.19% to 37.68%. The discharge ratio of Jianli and Chenglingji Stations raised from 1.36 to 1.61. The water level at all of the three stations decreased, especially at the Luoshan Station where the mean water flow and water level in July and August decreased by 6401.39 m3/s and 1.32 m, respectively. The hydrological correlation between the three stations decreased; however, a relatively stronger correlation (correlation coefficient is 0.9219) exists between the water flow in Luoshan Station and the water level in Jianli Station. The inter-jacking influence, though decreased at all rivers and lakeswith different magnitude of water level/flow, still exist, especially prominent in the effect of the high-water level at Luoshan Station on the outflow discharge at Chenglingji and Jianli Stations.