Abstract: The backwater effect caused by the confluence of the Dongting Lake is a critical factor influencing the hydrological processes in the Jingjiang reach, and an analysis of its changing characteristics is of critical importance for studying flood control safety issues in the middle and lower reaches of the Yangtze River. A quantitative method was proposed to analyze the degree and range of the backwater effect in terms of stage-discharge rating curves in the Jingjiang reach and the discharge ratios of the Dongting Lake inflow from 1990 to 2020. Then, a random forest regression model was constructed to calculate the backwater effect and sequence its four key influencing factors. The results demonstrated that: ① There was a significant positive correlation between the degree of backwater effect and the discharge ratio. In addition, the degree of backwater effect also increased with an increasing discharge from the mainstream. The mean degrees of backwater effect characterized by the water level variation were 0.59, 1.33 and 1.60 m respectively at the Jianli hydrometric station under the conditions of low discharge, bankfull discharge and flood discharge in the Middle Yangtze River during the period 2003—2020. ② The range caused by the backwater effect extended to the upstream with increasing mainstream discharges and discharge ratios. In 2020, with the increasing discharge ratios, the range zones under the three characteristic discharges were the reaches of Shishou—Shashi, Shishou—Chenjiawan and Chenjiawan—Zhicheng, respectively. ③ Furthermore, the contributions of discharge ratio, cumulative scouring depth, water level at the Luoshan hydrometric station, and mainstream discharge to the backwater effect accounted for 28%, 27%, 25% and 20%, respectively. It is varified that the random forest regression model can reflect the degree and range of the backwater effect under various discharge conditions in the Jingjiang reach.