Hydraulic fluctuation generated by the operation of the project would superpose in the lower approach channel of the Three Gorges Project. To explore the superposition law, we established a mathematical model of the lower approach channel of the Three Gorges Project and the river course between the Three Gorges Dam and the Gezhouba Dam using Preissmann four-point implicit difference scheme, based on the basic equations of unsteady flow in open channel. In this study, the superposition law of water level fluctuation in approach channel was studied, and the influence of channel base flow and initial water level on fluctuation characteristics was analyzed. Under the conditions of constant flow rate or discharging openly of the Gezhouba Project, the regression analysis of water level fluctuations at the lower approach channel entrance of the Three Gorges Project and the lower head of the ship lift was carried out. The results indicated that the water level fluctuation at the lower approach channel entrance resulted from the superposition of the average water rise caused by the net flow between the two dams and the oscillating process around the average water rise. The water level fluctuation at the lower head of the ship lift resulted from the superposition of the water level fluctuation at the lower approach channel entrance and the reciprocating flow wave in the lower approach channel. The attenuation degree of the relative fluctuation amplitude decreased exponentially with time. The expression obtained from the regression analysis could effectively obtain the fluctuation process of water level in the approach channel. The model had high precision.