The relationship between the water levels in Dongting Lake and Chenglingji station was investigated using long-term hydrology data record and the hydraulics theory. The mechanisms of the changing water level relationship between the two regions were discussed, and the critical condition in the changing process was analyzed. Furthermore, the impact of the Three Georges Project on the water level linkage was examined. Theoretical analysis showed that the relationship between the water levels in Dongting Lake and Chenglingji station agreed with the monotonic exponential function under fixed discharge. Considering distance and morphology of the lake bed, an empirical model was established to estimate the water level in Dongting Lake. Based on the empirical model, the water level variations of the Dongting Lake was estimated all throughout the ranges of inflow discharges and water level stages at the lake outlet by extrapolation from the observed data. Based on these calculations, rating curves of the water level relationships between Dongting Lake and Chenglingji station were established with lake inflow as parameters. Therefore, it was found that the combination of inflow discharge and water level at Chenglingji station can be divided into three types, as follows:no-effect zone, influencing zone, and determining zone in the coordinate plane. Based on the definition of critical condition, the threshold lines between the three zones were quantitatively determined. The water level estimation methods in different parts of the lake under the conditions of the three zones were proposed separately. Finally, the impact of the Three Georges Project on the critical condition and threshold lines were evaluated along with the observed data. The hydraulic linkage between Dongting Lake and Chenglingji station slightly changed after dam impoundment.