Water area, water level and water storage of reservoirs are fundamental data for water resource management. The method of satellite-based remote sensing has been demonstrated as an effective method for the estimation of water area, water level and water storage. However, the different applicability of the remote water extraction method and the limited time coverage of altimetry data bring difficulties to the study of the spatiotemporal variation characteristics of reservoir water area, water level, and water storage and their response relationships. Although many open-source data of reservoir water area and reservoir water storage at regional- and global-scale were published, these data were still hard to satisfy the long-term and high-frequency remote estimating reservoir dynamics due to its limitation in spatiotemporal resolution. Xin'anjiang Reservoir (Lake Qiandao), the largest man-made reservoir in eastern China, is a popular tourist destination and an important drinking water source in the Yangtze River Delta region. Here, we used the water index to extract the water area of the Xin'anjiang Reservoir from 1987 to 2022 with Landsat series data (1987-2022) and Sentinel-2 data (2019-2022), established response relationships of water level-water area, water level-water storage, and water area-water storage based on in-situ long-term water level and water storage data, and explored the causes of spatiotemporal changes in water area with land use data and hydrometeorological data. The results showed that: (1) The optimal water indexes for the Landsat series data and Sentinel-2 data to extract water extent in the Xin'anjiang Reservoir were AWEI_sh and GNDWI, respectively. Specifically, the F1-score are 91.93%, 91.03% and 93.14% for Landsat 5, Landsat 8 and Sentinel-2, respectively. Our method showed the highest F1-score value (91.26%) than the open-source dataset, GSWED (32.61%), JRC GSW (76.17%) and ReaLSAT (69.76%). (2) Temporally, the water area of the Xin'anjiang Reservoir had exhibited an upward trend from 1987 to 2022 (R²=0.20; P=0.01), with an overall growth rate of 0.96 km²/a. Spatially, permanent water bodies (with a water inundation frequency greater than 75%) accounted for 73.44% of the water area, primarily distributed in open areas. Seasonal water bodies (with a water inundation frequency greater than 25% and less than or equal to 75%) accounted for 10.17% of the water area, primarily distributed in the lake branch area. (3) The third-degree polynomial function showed the best performance in fitting the response relationships of water level-water area, water level-water storage, and water area-water storage in Xin'anjiang Reservoir. (4) The main factors causing dynamic changes in the water area of the Xin'anjiang Reservoir were precipitation in the upper reaches of the Lake Qiandao basin and land use changes caused by human activities.