The invention relates to selenium nano-particles and a preparation method thereof, in particular to amorphous selenium nano-particles with the grain diameter being 1-300 nm and a preparation method of the selenium nano-particles. The preparation method of the selenium nano-particles includes the steps that selenium combined macromolecules, a selenium source and a reducing agent or an oxidizing agent are subjected to a mixed reaction in a water phase medium, and nano-particles with the grain diameter being 1-300 nm are prepared; the selenium combined macromolecules are selected from water-soluble protein, water-soluble cellulose and water-soluble pectin and a combination of water-soluble protein, water-soluble cellulose and water-soluble pectin. According to the technical scheme, the nano selenium particles can be well stabilized, the nano selenium particles are not easily gathered into micron fine particles or larger selenium particles, and it is guaranteed that the nano selenium particles have good bioavailability and a good biological effect.