Subtropical China has a large area of Acacia mangium plantations; however, some key aspects of the structural and functional changes and the associated mechanisms after planting are still not well understood. We established a permanent plot in an A. mangium plantation and monitored the plant diversity (i.e., the number of species), biomass, soil physical and chemical properties and light transmittance from 1984 to 2018. Protocols for standard observation and measurement of Chinese Ecological Research Network were adopted. The total number of species in the plantation increased gradually from 3 to 38 during the 34-year period. The biomass in the tree layer initially increased rapidly from 1.60 to 185.01 t/ha within 7 years and then slowly increased to 188.69 t/ha during the following 27 years. The soil fertility of the A. mangium plantation improved continuously over the 34-year period. Vegetation restoration had positive effects on soil properties, such as soil moisture, soil bulk density, soil organic matter and soil total nitrogen and available phosphorus contents. The structure (i.e., plant diversity and light transmittance) and function (i.e., biomass and soil fertility) of the A. mangium plantation were not restored synchronously after 34 years of development. However, considering that the tree species richness in the natural forests in this region was 134, this plantation has slowly transformed from plantation to a natural forest. Artificial intercropping with native tree species can facilitate succession to a natural forest.