The runoff in the three outlets of Jingjiang River had an important impact on water resources and river and lake ecosystem security in Lake Dongting area. Due to the influence of human activities and climate change, the runoff in the three outlets of Jingjiang River had changed significantly. Based on the observed hydrological data, the characteristics of runoff in the three outlets along Jingjiang River was analyzed, and the influence of water supplement of the reservoir on the cutoff in the three outlets along Jingjiang River was evaluated by numerical simulation. The results indicated that the annual runoff in the three outlets along Jingjiang River showed a decreasing trend from 1981 to 2020, whereas the diversion ratio showed no significant change after 2003. Compared with 1981 to 2002, the runoff of the three outlets of Jingjiang River from 2003 to 2020 decreased by 26% and 40% in the flood period and the decreasing stage period, respectively, while that of the dry period and the rising stage period increased by 194% and 17%, respectively. The number of days of long-break and the frequency of short-break both increased for Shadaoguan station after 2003. For Mituosi station, the number of long-break days decreased but that of short-break increased. The number of days of long-break for Kangjiagang and Guanjiapu stations increased although the number of days of short-break increased with decreased frequency. In the dry year, the number of days with increasing discharge was only 1.2 times of the days which were influenced by the replenishment, with the replenishment efficiency no more than 0.28%. Hence, we do not recommend to postpone the cutoff of Maituosi and Guanjiapu stations by increasing the discharge of the reservoirs.