Selenium speciation in root soil samples collected from the Poyang Lake basin of Jiangxi Province was investigated in this paper. The results show a significant positive correlation between the species and total amounts of selenium in root soil. The residue species of selenium occupies 12.32% of total selenium, and the species of bound-state humic acid approximately accounts for 66.23% of the total amount of selenium in root soil. Meanwhile, selenium abundance in root soil insignificantly varies with the change of pH value but is significantly influenced by organic content. It is held that the process of selenium adsorption in soil organic substance and clay mineral causes selenium enrichment in root soil. This paper has proposed a method for estimating speciation concentration of selenium according to the total selenium and established a formula to calculate available selenium content with given data of selenium abundance and speciation in soils. In addition, a suggestion is put forward that only selenium species of organic bound, residue, water soluble and total amount of selenium should be considered in soil selenium assessment.