Functional diversity is a component of biodiversity reflecting the value and range of functional traits of species in a community or ecosystem. It can be used effectively to measure, explain and predict ecosystem function. Although many quantitative methods of functional diversity have been developed, the results of these methods are quite inconsistent and it is difficult to conduct a comparative study among them. Comparing and discussing the advantages and disadvantages of the main quantitative methods of functional diversity could be beneficial to expand the connotation of functional diversity and to promote its application and development. In this study, we reviewed 10 quantifying research methods of the functional diversity and examined in which way the functional diversity could be employed to explain and predict the effects of organisms on ecosystem functions, thereby providing a mechanistic link between the individual and the community/ecosystem scale. The key to develop effective methods for measuring functional diversity is to consider which species, what and how many traits should be used and weighted, among other considerations. By comparing the advantages and disadvantages of the 10 methods, we suggested that the measurement of functional diversity should meet the following criteria:(1) It can basically represent the functional diversity characteristics of the community/ecosystem; (2) It can calculate the qualitative, semi-quantitative and quantitative type of functional trait data; (3) It has no strict limitation for the trait data, e.g., the missing value and non-monotonous data should be allowed in the dataset; (4) Multiple functional traits data can be considered or calculated simultaneously; (5) The influence of species weight in the community/ecosystem can be considered; (6) Different communities/ecosystems can be compared by the method; (7) The intraspecific variance of functional traits can be considered; (8) The calculation is simple, easy to understand, and practical. Among the 10 functional diversity measurements, the functional dispersion index and the Rao's quadratic entropy coefficient have great potential to be applied, although they currently have some theoretical issues. The standardization of calculation method of functional diversity clearly needs further improvement in the future. Although new theories have been developed to predict ecosystem function with functional diversity, due to the methodological limitations, the application of functional diversity to study the structure and function of community/ecosystem (such as the research on the relationships between functional diversity and ecosystem productivity, stability, and resources dynamic) is still insufficient, and more field and experimental confirmation in this aspect are also urgently needed.