Abstract: Since the impoundment of the Three Gorges hydroproject, the decline of the low-water level along the Zhicheng river reach near the dam downstream of the Gezhouba hydrocomplex is small, but the falling amplitude of the low-water stage along the Yichang-Yidu river reach has greatly decreased. And the changes of the low-water level show the characteristics of non-uniform decline with time. This paper analyzes the time-variation characteristics of each factor in order to reveal the causes of this phenomenon, on the basis of the prototype observed data, including the water level, topography and composition of river bed of the Yichang river reach and its downstream, and a separation of variables method is applied to analyze the mechanism of the non-uniform drop in low-water level. The analysis results show that the falling process of the low-water level along the Yichang river reach was different before and after 2008. The decline in the low-water level along the Yichang river reach was small and slow down before 2008. After 2008, the falling amplitude of the low-water level increased and its falling speed was quicker. The scour from the Yidu reach to the Zhicheng reach is the main cause of the decline of low-water level in the Yichang river reach, and the deformation of the riverbed above the Yidu reach and the decline of water level in Zhicheng reach are the secondary causes. The adjustment of riverbed resistance caused by riverbed material armoring was a main cause of the decreasing amplitude of the low-water level along the Yichang river reach before 2008. After 2012, there was a little room for river bed deformation and resistance adjustment in the Yichang to Zhicheng river reach. The stability of the low-water level along the Yichang reach depends mainly on the fluctuation of water level along the Zhicheng river reach.