Based on a high-precision water surface dataset, combined with water system quantitative characteristics, structural characteristics and connectivity indicators, a quantitative analysis was conducted on the changes in water system pattern and connectivity during the flood season and dry season of the Poyang Lake Basin from 1995 to 2021. The results indicate that the total length of the main rivers in the Poyang Lake Basin remains largely unchanged during both the flood and dry seasons. However, there is a phenomenon where lower-grade rivers evolve into higher-grade rivers in the tributaries. The length of Grade I rivers in different hydrological periods in 2021 showed opposite characteristics compared to 1995. The overall increase rate of Grade I river length during the flood season reached 7.81%, while the decrease rate during the dry season reached 5.83%. The water system pattern of the Poyang Lake Basin during the flood season has become more complex, with overall increases in the number of water systems and structural characteristics. The growth rates of water surface ratio, river network density, river network complexity, and river network development coefficient all exceeded 5.5%. During the dry season, the development level of water systems remained largely unchanged, but the total area of rivers and lakes gradually decreased. During the flood season, the number of nodes and river chains in the river system generally increased(growth rate exceeded 8.5%), while during the dry season, the number of nodes and river chains remained stable overall. The water system connectivity remained basically unchanged during both hydrological periods, showing good connectivity.