Over the past 10 years, ecosystem functional diversity has played an important role in ecological research. Many studies have revealed that biodiversity has a substantial impact on ecosystem function through functional diversity, i.e., functional diversity is the factor closely related to the ecological process of biological diversity. The diversity of forest communities in the upstream and middle reach of the Wenyu River watershed, Shanxi was studied based on the field survey data. In the forest communities, we determined the growth form, life history, types of nitrogen fixation, C3/C4 type, light tolerance, leaf type, flower type, pollination mode, fruit type, seed propagation, flowering time, flowering phase, fruitage time and fruit season. These variables were used as plant functional traits to calculate species diversity indices, including richness index (R), species diversity index (H'), and evenness index (E), as well as functional diversity indices including the functional richness index (FRic), functional evenness index (FEve), and functional divergence degree index (FDiv). Classification of forest communities was analyzed using TWINSPAN and the correlation among diversity indices and between diversity indices and environmental factors were determined using Spearman's rank correlation analysis. The results indicated that:1) Based on the TWINSPAN classification results and in accordance with the principle and method of classification of vegetation, 52 forest quadrats in the upstream and middle reach of the Wenyu watershed were classified into nine formations, including Form. Betula platyphylla, Form. Picea meyeri, Form. Larix principis-ruprechtii, Form. Quercus wutaishanica, Form. Quercus wutaishanica+Pinus tabuliformis, Form. Picea wilsonii, Form. Populus davidiana+Betula platyphylla, Form. Populus davidiana and Form. Pinus tabuliformis. 2) The number of species was greatest in Form. Picea wilsonii (R=27) and Form. Quercus wutaishanica+Pinus tabuliformis, whereas Form. Pinus tabuliformis had the least (R=16); the H' and E of Form. Betula platyphylla were the largest, and they were smallest for Form. Pinus tabuliformis. 3) The FRic of Form. Populus davidiana+Betula platyphylla was largest, and it was smallest for Form. Picea meyeri; the FEve of Form. Picea wilsonii was the largest, and it was smallest for Form. Populus davidiana+ Betula platyphylla; the FDiv of Form. Populus davidiana was largest, and it was smallest for Form. Betula platyphylla. 4) There was no correlation between the forest community species diversity index and functional diversity index (P > 0.05), with only FDiv and H' being significantly negatively correlated (P < 0.05). 5) The correlation between the species diversity indices was highly significant (P < 0.01), the correlation between the functional diversity indices was not significant (P > 0.05), and FRic and FEve were significantly negatively correlated (P < 0.05). 6) The R value and altitude were highly significantly positively correlated (P < 0.01), the H' value and altitude were significantly positively correlated (P < 0.05), the H' the E values were significantly negatively correlated (P < 0.05), the FDiv value and latitude were highly significantly negatively correlated (P < 0.01), the FDiv value and altitude were highly significantly negatively correlated (P < 0.01). 7) As altitude increased, the species diversity index increased, and the linear relationship between the R and E values changed with altitude and was significant (P < 0.05); the linear relationship between the H' value and altitude was highly significant (P < 0.01), but as the functional diversity index decreased, the linear relationship between the FRic and FEve values changed with altitude was not significant (P > 0.05); the linear relationship between the FDiv value and altitude was highly significant (P < 0.01).