Based on a 21 King ETM+ remote sensing image, time series data of water level variation were interpreted. The water level-area function was derived using the measured water level data of the Sanhe Sluice, a new water level-capacity curve of Hongze Lake was obtained with the integral method, and the water level-capacity curve was compared with the existing curve. The results show that the culture area of the captive polder around Hongze Lake was formed early, and the captive polder condition was improved from 2000 to 2015, but shows an aggravation trend in the western part of Hongze Lake. The selection of the water level-area function has an influence on the calculation of the water surface area with a lower water level, resulting in an error in the calculation of the dead storage of the lake. When the remote sensing image data are more abundant, the difference between lake water levels in different images is smaller, and the water level-capacity curve is more accurate.