[Objective] The inter-annual time-series pattern of water quantity change and the seasonal difference of abundance, flatness and dryness in the Chaobai River Basin upstream of the Miyun Reservoir were studied to provide decision support for efficient water utilization and ecological water security in the Beijing-Tianjin-Hebei region. [Methods] Based on the collection of water quantity data and the analysis of the monthly wet water period, normal water period and low water period, the distance flatness percentage was used to explore the characteristics of water quantity changes in the Chaobai River basin, and to derive the quantitative relationship between the abundance, flatness and dry periods of the basin and the amount of runoff water. [Results] In the past 12 years, the dry season time of the Chaobai River basin is much longer than the wet season and the flat season time, but the high-water volume and its anomaly percentage will show an exponential growth trend every 1-2 years, indicating that the Chaobai River basin has more extreme climate characteristics in recent years. The high-water period of the Chaobai River basin mainly occurs from July to December, and the normal water period of the basin is basically accompanied by the high-water period. However, in the past 12 years, the normal water period of the Chaohe River basin and the Baihe River basin has only occurred for 6 and 7 months respectively; The low water period covers most of the 12 years, and it is densely distributed in all periods except the high-water period and the normal water period. In all periods, the water volume of the Baihe River basin is higher than that of the Chaohe River basin, which may be attributed to the recharge of runoff by snow melting in spring. Based on the fine division of flow and wet and dry periods, the quantitative relationship between the wet and dry periods and water volume in the Chaobai River basin is established. The proportion of the wet and dry periods is the most important factor affecting the water volume in the basin. [Conclusion] The Chaobai River Basin has experienced an increase in extreme precipitation events in recent years, but the percentage of low water periods is much higher than normal water period and low water period。The relationship between the proportions of the three water periods is the most important factor influencing the change in water quantity in this watershed.