Afforestation is gaining global attention for its role in carbon sequestration and timber production. Afforestation affects hydrological processes in forest ecosystems, including water yield. However, there is a paucity of longterm observation data to examine the effects of afforestation on water yield. Moreover, drought caused by global warming could change the runoff generation process and make the forest-water relationship hotly debated. To fully assess these effects, we used 22-year data from a long-time observation from 1997 to 2019 (excluding missing data in 2017) in a Chinese fir (Cunninghamia lanceolata (Lamb) Hook.) plantation in subtropical China. We analyzed the response of runoff to drought and stand growth post afforestation. Average runoff during drought declined by 41.2%, 37.5%, and 3.7% lower than nondrought events at monthly, seasonal, and yearly timescales, respectively. The drought affected runoff significantly when a drought event lasted 1.2 months, and drought severity was larger than 1.5. Annual runoff and runoff anomaly decreased with stand age during the first 14 years after afforestation and thereafter started to recover. A similar change pattern was found for the relationship between annual runoff and stand biomass. Annual runoff also decreased with an increase in the basal area. Monthly runoff was primarily controlled by rainfall and soil water rather than stand characteristics, while annual runoff largely depended on stand biomass. These findings clarify the relationship between planted forests and water resources and provide insightful information for sustainable water management in forests under global climate change.