Abstract: Microbes can convert oxidized toxic selenium into non-toxic selenium nanoparticles(SeNPs), this "green synthesis" method has great application potential in functional product development, environmental governance, and medical treatment. In this study, strain F4 was isolated from soybean planting bases with long-term application of selenium fertilizer. It was tested for selenium tolerance, identification classification, growth reduction kinetic analysis. In order to understand the mechanism of selenite reduction and elemental selenium formation, as well as its intracellular and extracellular distribution, the location of the selenite reduction reaction in bacterial culture were studied by in vitro reduction localization experiments. The results showed that, the strain F4 belonged to Bacillus altitudinis. When the culture was supplemented of 50 mmoL/L sodium selenite, the bacterial colony survival rate was 37.08%, and the tolerance to sodium selenite was as high as 250 mmoL/L. In the selenium-containing medium, the strain grew then reduced sodium selenite in the middle and late logarithmic phase, and produces a red precipitate in the stable phase. After incubating the reaction system for the in vitro reduction localization test at 33 ℃ for 24 h, reactivity was observed in the culture supernatant and extracellular polysaccharide, and the cytoplasm changed slightly with the addition of NADH. This bacillus could be used as a strain for producing red nano-selenium. It could provide a reference for the biotransformation of selenium.