The relationship between species diversity and functional diversity is currently one of the hot topics in ecology research. Determining the relationships between species and functional diversity in typical ecosystems of different regions can contribute to the development of biodiversity conservation theory. In this study, the main grassland ecosystems on the Qinghai-Tibetan Plateau, an alpine meadow and an alpine steppe, were selected to examine the relationships among species diversity, functional diversity, and primary productivity by using four species diversity indices, including the Patrick richness index, Shannon-Weiner diversity index, Pielou evenness index, and Simpson dominance index, as well as nine functional diversity indices, including the FAD functional trait distance index, MFAD functional trait mean distance index, FDp functional tree map index based on plots, FDc functional tree map index based on community, FRic functional volume index, FEve functional evenness index, Rao functional division two-times entropy index, FDiv functional division index, and FDis dispersion index. The aims of the study were to answer the following three questions:(1) what are the differences in plant species diversity and plant functional diversity between different types of alpine grasslands? (2) what are the relationships between plant species diversity and plant functional diversity in the alpine grasslands? (3) are there differences between the species diversity-ecosystem functioning (primary productivity) and plant functional diversity-ecosystem functioning relationships in the alpine grasslands? The results indicated that the alpine meadow showed higher species diversity, functional richness, and functional division than that by the alpine steppe. Furthermore, in the alpine meadow, the Patrick richness index was highly correlated with the functional indices FAD, MFAD, FDp, and FDc, as well as with the FDiv functional division index, and their correlations could be respectively expressed using a power function and two polynomial functions. In the alpine steppe, the Patrick richness index was strongly correlated with the functional indices FAD, MFAD, FDp, FDc, and FRic, and the Shannon diversity index and Simpson dominance index were highly correlated with the functional evenness index FEve; their correlations could be expressed by two polynomial functions. Moreover, the Pielou evenness index was highly correlated with the functional evenness index FEve, and their correlations could be expressed as an exponential function. Primary productivity of the alpine meadow was highly correlated with the Patrick richness index and FDiv functional division index, whereas primary productivity of the alpine steppe was highly correlated with all four biodiversity indices and the FDiv functional division index; all of their correlations could be expressed by two polynomial functions. Based on the results of this study, it can be concluded that plant species diversity, plant functional diversity, and their relationships with each other and with ecosystem function of primary productivity all varied between the different types of alpine grasslands. Therefore, in order to accurately evaluate the ecosystem functions of the alpine grasslands on the Qinghai-Tibetan Plateau, it is imperative to not only measure the plant species diversity but also the plant functional diversity, which is highly correlated with ecosystem function.