The invention discloses a preparation method of a lanthanum hydroxide nano particle phosphorus adsorbing material.Firstly ,lanthanum atom is generated by arc excitation method using lanthanum metal aselectrode, and the lanthanum atom collides and merges to form lanthanum nanoparticle in the process of inert gas carrying, and then the lanthanum nanoparticle reacts with water vapor at normal temperature and pressure to generate lanthanum hydroxide nanoparticle. Finally, the carrier material is used. The lanthanum hydroxide nanoparticle phosphorous adsorption material was obtained by receiving lanthanum hydroxide nanoparticle. The synthesis process is simple, rapid and no by-product is produced. It is mainly used for the adsorption of phosphorus in water through the adsorption of phosphate.