Natural enclosure refers to the forest in a closed state, which is extensively believed to be an important and effective approach to cultivating forest resources. Fine roots, as the important resource-acquiring organ of plants, are crucial to the ecological function fulfillment of forest communities. To clarify the effects of natural enclosure on the fine root functional traits, the moso bamboo (Phyllostachys edulis) forests under human management and enclosure for 3, 5, 10, 15 years, and 40 years in the Wuyi Mountain National Park were chosen as the research objects, using the method of substituting space for time, to compare the differences in the fine root functional traits of moso bamboo and investigate their adaptations to the changes of soil physical and chemical properties under different enclosure years. The results indicated that: (1) The natural enclosure had a significant effect on the functional traits of moso bamboo fine roots. With the increase of the enclosure year, the specific root length (SRL), specific root area (SRA), total carbon (C), total nitrogen (N) and total phosphorus (P) content of moso bamboo fine roots showed a trend of increasing and then decreasing, and root tissue density (RTD), the C ∶ N and C ∶ P ratio showed a trend of decreasing and then increasing. Compared with other natural enclosure forests, the SRL, SRA and nutrient content of fine roots in the moso bamboo forest community enclosure for 10 and 15 years were significantly improved. The soil layer and diameter class had significant effects on fine root functional traits equally. (2) There was a significant correlation between the functional traits of moso bamboo fine roots with the changes of enclosure year. SRL, SRA and nutrient content had significantly positive correlations, and they had significantly negative correlations with C ∶ N, C ∶ P and RTD. (3) With the successional process of enclosure, the changes of soil factors in the moso bamboo forest community drove the changes of the fine root functional traits. There was a significantly negative correlation between soil capacity and fine root nutrient contents. The C and P content of soil were significantly positively correlated with the SRL and SRA of fine roots. The soil nutrient contents showed a highly significant positive correlation with the fine root nutrient contents. In conclusion, the natural enclosure prompted the fine roots to reasonably weigh the allocation of resources between different functional traits through their phenotypic plasticity. After 10-15 years of the natural enclosure, the moso bamboo fine roots concentrated more nutrients on the construction of resource-acquisitive traits, with significantly higher absorption capacity and efficiency, the stand productivity and ecological functions being superior. Appropriately scientific human management activities after 10-15 years of natural enclosure would contribute to the sustainable and healthy development of moso bamboo forests.