Sonneratia apetala was introduced to China's Hainan Island from Bangladesh in 1985. In recent years, because of its adaptability and rapid growth, S. apetala has become the most important species to ensure the forestation of mangroves that are imported to the coastal and intertidal regions of South China. This study was conducted to explore the ecological responses of understory macrofauna communities during the transformation of a pure S. apetala plantation and the major environmental factors affecting them. Two pure plantations of 1-year-old S. apetala were chosen as the study sites: one was at Zhanjiang Island Mangrove Nature Reserve in Leizhou, and the other was at Qi'ao Island Mangrove Nature Reserve in Zhuhai, Guangdong Province, China. Indigenous mangrove seedlings of Rhizophora stylosa or Bruguiera gymnorrhiza were mixed with S. apetala by underwood planting. The macrofauna communities of the transformed 1-year-old S. apetala + R. stylosa and 1-year-old S. apetala + B. gymnorrhiza mixed plantations were compared with those of a similarly aged pure S. apetala plantation. The results showed differences in the dominant species among the macrofauna communities in the 1-year-old pure S. apetala plantation and the 1-year-old S. apetala + R. stylosa and 1-year-old S. apetala + B. gymnorrhiza mixed plantations. The results of a one-way analysis of similarity, hierarchical clustering, and nonmetric multidimensional scaling all showed significant structural differences among the macrofauna communities of the pure plantations and mixed plantations, which had similar forest ages. In addition, the results showed that transforming pure S. apetala plantations by underwood planting with seedlings of R. stylosa or B. gymnorrhiza may improve the biomass and species diversity of the understory macrofauna. BIOENV analysis showed that macrofauna distribution was closely related to the amount of indigenous mangrove plant litter of R. stylosa or B. gymnorrhiza, which further confirmed that underwood planting with indigenous mangrove species played an important role in the distribution and species diversity of the understory macrofauna. Because the levels of the physicochemical factors in the soil in the three forest types were not significantly different and the upper canopy of these forests was entirely comprised of S. apetala, it can be speculated that underwood planting with R. stylosa or B. gymnorrhiza increased the understory indigenous mangrove plant litter, the nutrient levels (such as the C/N ratio), and the tannin content that may be adaptive or preferred by indigenous macrofauna. Furthermore, some mobile mollusks and crustaceans move to mangrove habitats that are more suitable for survival, leading to a rise in the biomass and species diversity of understory macrofauna in mixed plantations. However, experimental results from the two research sites showed that the effect of underwood planting in pure S. apetala plantations with B. gymnorrhiza was superior to that with R. stylosa because the average seedling height, litter production, percentage of litter production accounting for the communities' total litter production, and the survival rate of seedlings were all greater for underwood planting with B. gymnorrhiza than with R. stylosa. This also resulted in a higher biomass increase and species diversity of macrofauna in the underwood planting with B. gymnorrhiza than with R. stylosa. This experiment was conducted 3 months after underwood planting with R. stylosa or B. gymnorrhiza in the pure S. apetala plantations. Since the total research period was only 1 year, this study only explored the short-term effects of the experiment; therefore, future studies on its long-term effects are required.