As the area of plantation forests steadily expands, the diversity of understory species plays a significant and irreplaceable role in multiple aspects, such as water conservation, soil and water retention, and the enhancement of the stability of forest ecosystems. Maintaining and promoting understory species diversity are crucial for the healthy development and sustainable operation of forest ecosystems. In order to explore the key factors influencing the understory species diversity, this study took the Larix principis-rupprechtii plantation forest, Pinus tabuliformis plantation forest, Pinus sylvestris plantation forest, and L. principis-rupprechtii×Betula platyphylla mixed plantation in the Qilaotu Mountains as the research objects. These forest types are representative plantation forests in this region, and research on them can provide valuable insights into the understory species diversity in similar forest ecosystems. This study employed a series of scientific research methods, including one-way analysis of variance, Pearson correlation analysis, stepwise regression analysis, and variation partitioning analysis. The results of the study were as follows: (1) The understory species diversity of the four typical plantation forests was abundant. In total, 68 species were recorded, which were classified into 56 genera across 43 families. Among these, the dominant families in the understory of the plantation forests were mainly Asteraceae, Rosaceae, Ranunculaceae, Poaceae, and Brassicaceae. These families were common in the understory of plantation forests and play important roles in maintaining the balance and functionality of the forest ecosystem. (2) Among the four forest stand types, L. principis-rupprechtii×B. platyphylla mixed plantation exhibited the highest level of understory species diversity, encompassing 47 species from 45 genera belonging to 32 families. Moreover, there were significant differences in the species diversity index when compared to the other three forest stand types. This indicates that the mixed plantation forest provided a more favorable environment for the growth and coexistence of various species. (3) Through Pearson correlation analysis and stepwise regression analysis, it was found that within the forest stand structure, the diameter at breast height (DBH) was the key factor influencing species diversity. In addition, forest stand density, canopy density, and tree height were identified as the main factors affecting species diversity. These factors interact with each other and jointly influence the understory species diversity. (4) Further analysis through variation partitioning revealed that the forest stand structure had the highest explanatory rate. Specifically, it accounted for 49.1%, 44.6%, 50.3%, and 20.5% of the Shannon-Wiener diversity index, Simpson diversity index, Pielou evenness index, and Margalef richness index, respectively. This shows that the forest stand structure plays a dominant role in determining the understory species diversity. In conclusion, if we aim to increase the understory species diversity of plantation forests, it is necessary to appropriately adjust the DBH of the upper trees in the plantation forests. By doing so, we can optimize the forest stand density and canopy density, improve the forest stand structure, and ultimately enhance the understory species diversity. This study provides a theoretical basis and practical guidance for the management and protection of plantation forests and the promotion of forest ecosystem health.