Abstract: As (V) ion adsorbent was prepared by using diatomite as matrix, MnCl₂ and NaOH as the complex modifiers. The effect of adsorbent dosage on the removal rate of As(V) was studied. The samples of the adsorbent before and after the modification were analyzed by SEM, IR, XRD and XPS. The results show that the removal rate of As(V) increases from 44.9% to 93.9% with the increase of MnCl₂-NaOH/diatomite adsorbent. After modification, the diatom matrix is not changed, and the average pore size of MnCl₂-NaOH/diatomite adsorbent increases and the specific surface area increases by nearly seven times. After the diatom modification, the active hydroxyl peak disappears, and the new Mn-O-Si absorption peak appears on the surface. XRD data show that Mn-O-Si phase is newly formed. The surface of Mn gets electrons, through oxidation and diatom surface formation of MnO₂ composite grafting. The adsorption of As(V) ions on the adsorbent results in the formation of Mn₃ (AsO₄) (OH)₄. As in the adsorption process, As gets electrons, Mn losses electrons, while O and Si surface electrons shift to As. Adsorption on the surface of MnCl₂-NaOH/diatomite adsorbent is coordination adsorption, charge adsorption, and so on.