Changes in stand spatial structure affect herbaceous species diversity in forests. Our goal was to explore the relationship between stand spatial structure and herbaceous species diversity in the understory of forests and provide optimum conditions for maintaining and improving herbaceous species diversity in the forest understory. In this study, the effects of stand spatial structure on herbaceous species diversity were investigated in pure coniferous forests, mixed broadleaved and coniferous forests, and evergreen broadleaved forests. The contributions of stand spatial horizontal structure, vertical structure, and individual trees competition to the herbaceous species diversity were estimated. Results showed that the stand horizontal structure had a significant effect on the species diversity and species evenness of herbaceous species (P < 0.001), with influence coefficients of 0.96 and 0.89, respectively. The stand horizontal structure significantly affected herbaceous species richness (P < 0.01) with an influence coefficient of 0.22. The stand vertical structure had a significant effect on species diversity and species evenness (P < 0.001) and species richness (P < 0.01), with influence coefficients of 0.86, 0.43, and 0.16, respectively. Individual tree competition had a significant effect on species richness and species diversity (P < 0.001) with influence coefficients of -0.47 and -0.30, respectively. However, the competition of individual trees had no significant influence on species evenness (P > 0.05), and the corresponding influence coefficient was -0.04. Our results indicated that the herbaceous species diversity within the studied forests was primarily affected by the horizontal spatial structure of the stand, followed by the vertical spatial structure of the stand, and finally, individual tree competition in the studied forests, although these three stand structure variables had strong correlations with herbaceous species diversity. The study suggested that to maintain or improve the herbaceous species diversity in forests, comprehensive forest management should be adopted to mainly adjust the stand horizontal spatial structure. Additionally, regulated stand vertical spatial structure and appropriate reductions in the degree of individual tree competition in the forests should be included.