The evaluation of near-naturalness is a widely discussed topic at present. It is of great significance to restore forest ecological function and protect forest sustainable development by establishing a near-naturalness evaluation system. However, there is no unified evaluation system for forest near-naturalness. This study focuses on the construction of near-natural evaluation based on water conservation capacity and provides theoretical support for the restoration of forest ecosystem services. In order to explore the management mode of near-natural forests, this paper takes the artificial forest in the outer edge of Qilian Mountains in Qinghai as a case study. The forest in Qilian Mountains plays an important role for water conservation in western China. However, in the early stage, some plantations were planted according to the timber forest structure. As a result, there are widespread problems such as poor stability of forest system, low water conservation function, and difficulty in sustainable development of forests. It is an urgent problem to objectively evaluate the water conservation capacity of existing plantations and explore the near-natural management measures of forests. In this study, 40 spots planted with the Picea crassifolia Kom. forest, Betula platyphylla Suk. Forest, and Picea crassifolia Kom. and Betula platyphylla Suk mixed forest with different stand origins in Jinchangou and Talgou small watershed in the eastern Qilian Mountains were investigated. Sensitivity analysis was carried out on the indexes related to naturalness and water conservation ability, and the medium and high sensitivity indexes were selected. The forest water conservation function was evaluated quantitatively by fuzzy matter element method, and the near-natural degree of artificial forest was evaluated quantitatively by Euclidean distance method combined with the entropy weight method. Six methods were used for regression fitting (R²>0.4) of near-naturalness and water conservation scores, and four near-naturalness evaluation levels were established by K-means method. The results showed that most of the artificial forests in the eastern Qilian Mountains were in the semi-natural forest stage, and the near-naturalness and water conservation ability of the pure forests could be improved by transforming them into mixed forests. In the meanwhile, the study demonstrated that the forests in the semi-natural stage were the key object of forest transformation. By transforming semi-natural forests, the water conservation capacity of local forests can be greatly improved. In addition, the study still needs to investigate the near-natural forests to improve the progress of the evaluation system and further explore the near-natural forest management measures for semi-natural forests.