In the study of biological diversity, functional diversity can reflect the structure and function of the ecosystem better than species diversity. Therefore, more and more scholars advocate replacing species diversity with functional diversity in the study of community ecology. At present, the secondary pure Pinus massoniana forest is a type of forest that urgently needs to be transformed, and Cinnamomum camphora is one of the ideal tree species for P. massoniana forest transformation in sub-tropical regions. In order to improve the study on the transformation of P. massoniana forest to C. camphora forest and guide the transformation of P. massoniana forest in practice, four communities at different stages of the transformation of P. massoniana forest to C. camphora forest were selected to study the functional characters and diversity of the understory shrub layer and herb layer. The results showed that: (1) With the progress of forest transformation, the number of species and the diversity of functional traits of understory plants indicated a trend of increasing first and then decreasing, among which the functional richness and functional evenness peaked at the early stage of the transformation, while the number of species and functional divergence peaked at the middle stage of the transformation. (2) Compared with the four stages of forest transformation, the comprehensive performance of the functional diversity of understory plants was ranked as follows: early stage of transformed community > middle stage of transformed community > late stage of transformed community > untransformed community. (3) In the process of transformation, the variation trend of functional traits and diversity of "light response traits" and "reproductive traits" of understory plants was similar, but the former was more sensitive to the forest transformation than the latter. (4) In the process of forest transformation, the growth, development and diffusion of understory plants were affected by undergrowth light resources. With the species competition, the growth, development and diffusion of undergrowth plants were gradually manifested as the change of species diversity and spatial distribution pattern, which ultimately affected the function of the ecosystem. (5) The transformation from P. massoniana to C. camphora forest accelerated the succession process of the community and improved the function of the community. However, there were still problems such as uneven distribution of resources and decrease of functional diversity of plants due to the serious interference of trees on the light resources of understory in the late stage of the transformation. In the reconstruction of secondary pure forest of P. massoniana, suitable reforestation density and the model of mixed forest should be selected.