Northeast of Sichuan Province is a main part of Daba Mountains. There large area of old-growth beech forest dominated by Fagus engleriana is still reserved well. We studied species diversity of the plant community by investigating 4 plots consisting of 28 samples, with a size of 10m×10m for each sample. The main results are: (1) There are 197 vascular plant species belonging to 140 genera of 74 families in these samples. Tree species mainly belong to the families of Fagaceae, Rosaceae, Ericaceae, Betulaceae and Aceraceae. Species of Rosaceae, Caprifoliaceae and Ericaceae dominate the shrub layer. On the forest floor, common species belong to the families of Cyperaceae, Compositae, Liliaceae and so on; (2) On the composition of geofloristic elements, temperate element is most, with 65.38％ and 64.32％ in total genus and species numbers in the community, respectively. Tropical and subtropical elements are less than a quarter in both genus and species numbers. (3) The curves of species abundance nature logarithm vs species number are significantly different in the tree, shrub and herbaceous layers. (4) The species richness of shrub layer is highest, and that of tree layer is lowest. The Shannon-Wiener index, Simpson index and evenness index are highest in the herbaceous layer, and are lowest in the shrub layer. (5) The species diversities of three layers are directly affected by abundance and coverage of tree and shrub layers. This may indicate that species diversity of the beech forest is closely correlated to the structure characteristics of the community.