Peatland ecosystem functions largely depend on the functional traits of the species. Although the plant functional traits and functional diversity have been considered as the key parameters to predict the response of peatland carbon cycling to the climate change, the studies are still limited. Here, we systematically investigated the specific leaf area, specific root length, specific root area, and the carbon, nitrogen, phosphorus, and biomass contents of leaf and root tissues in four shrub-sedge peatlands of the Da Xing'an Mountains. The community weighted mean (CWM) value, function dispersion index, Rao's quadratic entropy index, and coefficient of variation value were used to estimate the functional traits of plants in the shrub-sedge peatland. Combined with the biodiversity indices, we analyzed the relationships among biodiversity, functional traits, and functional diversity. Results showed that: (1) The nitrogen and phosphorus contents of leaves of shrub (Betula fruticosa) were significantly higher than those of sedge (Eriophorumvaginatum), while the coefficient of variation of the specific leaf area of Betula fruticosa was lower than that for Eriophorumvaginatum; (2) The specific root length and the specific root area of the shrub fine roots were significantly lower than those of the sedge, while the nitrogen and phosphorus contents of the shrub fine roots were significantly higher than those of the sedge. In addition, the leaf and fine root biomass of the shrub was significantly higher than that of the sedge. (3) The functional dispersion index and Rao's quadratic entropy index of plants inhabiting in shrub-sedge peatlands were very low, and their coefficient of variation was higher compared to the indices of biodiversity and community weighted mean; (4) The relationship between above- and below-ground plant tissues traits of the shrub was different from that of the sedge. Shrub root length was significantly positively related to the root area. In addition, the carbon, nitrogen, and phosphorus contents of the sedge leaves and roots were positively correlated with each other. The relationship between the indices of functional dispersion and the biodiversity index was identical to that between Rao's quadratic entropy index and the biodiversity index. Furthermore, there was a strong coupling relationship between the functional dispersion index and Rao's quadratic entropy index. This study highlights that the shrubs were better adapted to the nutrient-poor environment of the peatlands in the Da Xing'an Mountains and there was a strong competition between plant species for the resource.