[Objective] This study aimed to clarify the differences in water conservation function of Pinus sylvestris var. mongolica stands at different ages.[Methods] We quantified the differences in water conservation functions of P. sylvestris var. mongolica stands at different ages（10, 15 and 20 years） based on 2023—2024 field observations of throughfall, stemflow soil water content, and litter water content. [Results] As stand age increased, the development of canopy structure significantly altered the pattern of rainfall distribution. Specifically, the proportions of throughfall and stemflow decreased, while canopy interception capacity continued to increase, highlighting the crucial role of mature canopies in redistributing rainfall. For instance, canopy interception accounted for 17.7%, 29.4% and 35.4% of rainfall in 10, 15 and 20-year-old stands, respectively. The understory litter layer also exhibited a significant age effect, with water-holding capacity increasing exponentially with the accumulation of litter, forming a synergistic effect with the water-holding function of the surface soil. The dominant role of soil water storage highlighted the stability of deep water reservoir, while differences in water storage characteristics among soil layers at different stand ages revealed dynamic vegetation-soil interactions. Water conservation function of the forest ecosystem showed a unimodal trend with stand age. Compared to 10-year-old stands, there were significant increases（61.6% and 56.0%）（p＜0.05） in 15 and 20-year-old stands. [Conclusion] The age-dependent variations in water conservation function can provide crucial references for future reforestation projects and sustainable management of P. sylvestris var. mongolica plantations.