The realization of the forest water conservation function is an important aspect of the ecosystem, contributing to human well-being. Some studies evaluated soil water conservation function based on the soil water storage capacity, while the actual water conservation and water storage capacity often have certain differences. It is not rigorous to directly equate the water conservation function of litter and soil with its maximum water capacity in quantitative analysis. In recent years, it attached great importance to the distinction between the water holding capacity and the water conservation amount of the forest ecosystem in study. In this study, we analyzed the spatial variation of forest ecosystem water holding capacity (maximum water capacity) and actual water conservation along the elevation gradient, and the results showed that the variation patterns were very different. In this study, we selected the Jiufeng national forest park, which has a representative vertical distribution of soil and vegetation in the soil-rock mountain area of north China, as the research site. We took elevation as the main spatial variable, monitored the main hydrological processes at 10 gradients stations within the elevation range of 150—1150m, and concluded that: the water holding capacity of the forest ecosystem basically depends on the water storage capacity of the soil layer (91.89%), which basically shows a monotonous increasing trend with the elevation, while the change of the actual water conservation is complex. Forest ecosystem actual water conservation presented the change of "decrease-increase-decrease", its decline (89—55mm) in the range of 150—450m is due to the evapotranspiration increase with elevation in this elevation interval, and evapotranspiration accounted for 80% of the precipitation. In the 75—1150m range, the decline in precipitation with increasing elevation is the main reason for the decrease in actual water conservation (from 130 to 88mm) in this elevation interval. The influence rate of the three water conservation layers on surface runoff: soil gravel content (40.59%) > litter volume (37.00%) > leaf area index (30.21%). High soil gravel content and concentrated rainfall in the study area facilitate maximum groundwater recharge, with deep soil water leakage accounting for 20% of the precipitation. Significant water infiltration is beneficial for groundwater replenishment and reduces the risk of soil erosion from heavy rainfall. However, the high gravel content of the soil causes the soil water deficit (-1.61%/a). Decreased soil water availability may limit vegetation growth, and insufficient water supply in the dry season may lead to the lack of ecological base flow in basin. Using elevation change as an indicator of the warming and drying climate trend in North China, we found that the water conservation function of the forest ecosystem in this area may weaken in the future, potentially increasing the ecological pressure from water scarcity.