Abstract: Water conservation of forestland is one of the most valuable service functions in the terrestrial ecosystem, especially in arid and semi-arid areas. It is in high demand to explore the evolution of spatiotemporal differentiation patterns in forestland and water conservation functions, as well as their relationship. Tree species structure collocation and forestland area layout can greatly contribute to the water conservation function under reasonable afforestation project management. Taking Zhangjiakou City as an example, the spatiotemporal differentiation characteristics of forestland and water conservation function were qualitatively analyzed using InVEST, and SPSS models, according to the remote sensing image data. The mutual relationship was also quantitatively measured from the two aspects of spatial distribution and quantitative correlation. The results showed that: 1) The distribution of forestland shared outstanding spatial agglomeration. The area of forestland continued to increase from 9 144.99 to 13 650.36 km², with an increase of 4505.37 km², and 49.27 % from 1990 to 2020. The largest area of broad-leaved forest was obtained from the area ratio, in terms of different tree species. By contrast, there was the smallest area of coniferous forest from the point of view of the growth rate. Among them, the shrubs grew fastest with a growth rate of 95.64 %, whereas, the coniferous forests grew the slowest, with a growth rate of only 36.02%. 2) The water conservation function was high in the east and south, while low in the center and west. The average water conservation decreased first, then increased, and finally decreased. 3) There was a consistent spatial distribution of forestland area and water conservation capacity. Water conservation increased first and then decreased with the increase in forest area. Furthermore, water conservation reached its peak, when the forest area reached 1/2 of the total area. The relationship between water conservation and forest structure was ranked the descending order of the broad-leaved forest > shrub > coniferous forest. 4) The area of economic forest and other forest species should be planted reasonably in the future. Much attention should be paid to the planting of mixed forests and primary forests, in order to realize the coordinated development of socio-economy and ecological protection. In addition, some findings were also needed to further explain: there was no increase in the water conservation function with the increase of forestland area. Therefore, the decision-making on ecological engineering projects can comprehensively consider various ecological service functions in the actual construction, rather than the increase of forestland area. Quantitative analysis can be made to determine the most reasonable forestland area and tree species collocation. In addition, it is very necessary to comprehensively analyze the influencing factors of the water conservation function from the perspectives of natural conditions, social economy, and regional policies, so as to promote the full play of regional water conservation capacity and the construction of water conservation functional areas and ecological environment support areas in Zhangjiakou City, Hebei Province of China.