[Objective] The interannual time-series patterns of water quantity changes and seasonal differences in wet, normal and dry in the Chaobai River basin upstream of the Miyun Reservoir were studied to provide decision support for efficient water utilisation 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, normal, and dry periods, 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 wet, normal and dry periods of the basin and the amount of runoff water. [Results] From 2010 to 2021, the dry season of the Chaobai River basin was much longer than that of the wet and normal seasons, but the high water volume and its anomaly percentage showed an exponential growth trend every 1—2 years, indicating that the Chaobai River basin had more extreme climate characteristics in recent years. The wet period in the Chaobai River basin mainly occured from July to December, and the normal period of the basin was accompanied by a wet period. However, from 2010 to 2021, the normal periods of the Chaobai River basin only had a normal water period of 6—7 months, respectively, and the dry period covered most of the 12 years, and it was densely distributed in all periods except the wet dry period and the normal period. During all periods, the water volume of the Baihe River basin was higher than that of the Chaohe River basin, which may be attributed to the recharge of runoff by melting snow in spring. [Conclusion] The Chaobai River basin experienced an increase in extreme precipitation events from 2010 to 2021; however, the percentage of dry periods was much higher than that during the normal and wet periods. The relationship between the proportions of the three periods was the most important factor influencing the change in water quantity in this watershed.