Abstract: A new concept of river-lake water exchange coefficient is introduced to investigate the river-lake water exchange in the middle and lower reaches of Changjiang River. The concept is constituted of the ratio of the amount of runoff from tributaries to a lake and the amount of runoff from the lake to the main stream of a river during a certain period of time. The equilibrium theory is used in the derivation of the river-lake water exchange coefficient. Based on the latter, the exchange process in river-lake water can be classified into three types: Diverting flood waters to lake, water balancing, and draining lake water out to river. Results show that the water exchange between the Dongting Lake and the main stream of Changjiang River has evolved from diverting flood waters to lake, to water balancing, and to draining lake water out to river during the past 60 years. No significant changes can be observed in water exchange types between the Poyang Lake and the main stream of Changjiang River, as revealed by fluctuations of the river-lake water exchange coefficient indicating mostly the water balancing type. The river-lake water exchange coefficient can be strongly correlated to the amount of runoff from the main stream of Changjiang River. In contrast, a week correlation is found in lake's tributaries. Thus, the river-lake water exchange is dominantly controlled by the water flow in the main stream of Changjiang River.