This paper analyzes main factors influencing the low water level after the impoundment of the Three Gorges project,calculates the designed stage of Jingjiang reach according to the hydrological data during year 1982 and 2002,and checks the stage referencing the predicted value obtained from the water-sand mathematical model and according to the low stage of Jingjiang reach after the impoundment,we get the designed stage which is relatively safe and which reflects not only the impact of the impoundment ,but also the variation trend of the designed stage.