The research on the water level–water area relationship model is time-consuming and inefficient resulting from the cumbersome data downloading and processing, massive use of professional software, and frequent manual intervention in the water extraction from long-term sequence remote sensing images. We preprocess the Landsat images from 2002 to 2016 on Google Earth Engine. Then, we identify the water of East Dongting Lake by NDWI and calculate its area. On this basis, with the daily water level data from Chenglingji Hydrological Station, we identify the optimal water level corresponding to the image, and draw the water level–water area curve by the weighted average water-level method, the threshold method, and the single-point water-level method. The results are as follows: (1) The water scope of East Dongting Lake varies greatly with season, with the largest area in between July and September and the smallest area in between January and March. (2) The accuracy of calculating the optimal water level by the threshold method (R² = 0.9628) is higher than that by the weighted average water-level method (R² = 0.8322) and that by the single-point water-level method (R² = 0.9457). (3) The fifth-order polynomial model has the highest accuracy (R² = 0.9628) in fitting the water level–water area curve.