The water level change in Poyang Lake is very complex, while traditional techniques are difficult to monitor such dynamic changes with large scale and high precision. In September 2018, the Ice, Cloud and Land Elevation 2 (ICESat-2) satellite, equipped with the world's advanced photon counting laser altimeter, was successfully launched, which provides a good opportunity to monitor the dynamic changes of water levels in medium and large inland lakes. In this study, the monthly ATL13 global inland water data product of ICESat-2 satellite from 2018 to 2020 are used to estimate and analyze the water level variation of Poyang Lake. The measured data of Hukou, Xingzi and Kangshan hydrology stations were used for verification and error correction, and the water level and rainfall data of each station were combined to analyze the dynamic variation of Poyang Lake water level and its causes. The results showed that: (1) The annual water level of Poyang Lake varied sharply with obvious seasonal variation, and the overall water level showed an increasing trend. The high-water level period was from June to October, and the low water level period was in other months, and the peak of water level appeared from July to September. (2) ICESat-2 has a high linear correlation with the measured water level with the coefficient of above 0.846, and the coefficient is as high as 0.974 after error correction. The root mean square error (RMSE) is 1.660m at Hukou station, 1.073m at Xingzi station, and 0.836m at Kangshan station. After error correction and recalcitration, the measurement accuracy is improved by nearly 1m, and the RMSE is 0.663m at Hukou station, 0.659m at Xingzi station, and 0.440 m at Kangshan station. (3) The variation of water level in Poyang Lake is highly correlated with the variation of rainfall. The precipitation is low in January-February and October-December, and the water level decreases in the dry season. From March to October, the rainfall increases, and the water level rises. Rainfall mainly occurs from July to September, when is the peak of water level.