Plants have many indispensable and important functional traits that help them respond and adapt to changing environmental conditions, optimize their functions, and even reproduce across generations. Plant functional trait diversity, also known as plant functional diversity, is a comprehensive index for characterizing the distribution range and dispersion of plant functional traits in a community. It reflects the overall differences, adaptation, and optimization mechanisms of functional traits in plant communities. Because they account for redundant species and interspecific complementary effects within communities, these functional diversity parameters are uniquely advantageous in predicting community mechanisms and the functional evolution of ecosystems. Since 2010, these concepts, their theory, and methodology have been progressively developed, and the application of functional diversity parameters is increasingly integrated into several important fields of ecological studies. However, the selection of suitable parameters for the different functional diversity is a major challenge for researchers to date. Furthermore, it is highly debatable whether the number of functional traits and their selection have an impact on functional diversity parameters. In this chapter, we review the definition of plant functional diversity, introduce its origin and significance, and systematically summarize several common functional diversity parameters and their algorithms, which include functional richness, functional evenness, and functional dispersion. We then present an analysis of the influence of the number of functional traits on functional diversity parameters at the regional scale, the changing patterns of plant functional trait diversity at the large scale, and their relationship with productivity. Finally, we explore the development direction of plant functional diversity to provide novel insights to upcoming researchers.