To understand the coupling relationship between forest community characteristics and species diversity, three typical communities (pure forest of Quercus acutissima, mixed forest of Q. acutissima with various broad-leaved trees, and mixed forest of Q. acutissima with Pinus tabuliformis), were selected at Qiaoshan, southern Loess Plateau, to measure the community characteristics (height, diameter at breast height, and coverage). According to these indices, important values and species diversity were calculated, and redundancy analysis (RDA) was performed. The results showed that (1) the height and diameter at breast height and clear tree bole height of the arbor layer of Q. acutissima with P. tabuliformis were the highest in the mixed forest; the shrub height, shrub crown width, shrub ground diameter, shrub coverage were the highest in the mixed forest of Q. acutissima with various broad-leaved trees; and the herb ground diameter, herb coverage, and herb crown width were the highest in pure forest of Q. acutissima. (2) In three different forests, the important value of Q. acutissima in the tree layer was maximal, and the dominative species in the shrub layer were Sophora viciifolia, Celastrus orbiculatus, and Lespedeza bicolor, and that in the herb layer was Carex tristachya. (3) The biodiversity index of trees and shrubs in the mixed forest of Q. acutissima with P. tabuliformis were the highest, while the diversity index of herbs in the mixed forest of Q. acutissima and various broad-leaved trees were the highest. (4) There were differences in the relationships between the community structure characteristics and species diversity of different types of Q. acutissima forests. In the mixed forests of Q. acutissima with P. tabuliformis, the DBH, new twig length, and height of the shrub had the highest impact on species diversity. In the mixed forests of Q. acutissima with various broad-leaved trees, the height and crown width of the shrub had the highest impact on species diversity. In the pure forests of Q. acutissima, the DBH had the highest impact on species diversity. (5) Tree diameter breast height, tree height, and species diversity in the Q. acutissima with P. tabuliformis forest and in the mixed forest of Q. acutissima with various broad-leaved trees were higher than that in the pure forest of Q. acutissima. We can conclude that these two forest types have high wood productivity and ecological protection capacity, and can be the direction of future forest management.