In this research,the removal efficiency of As (Ⅲ) by biofilter was studied under filtration velocity of 3,4 and 5 m/h respectively,using well-established biofilter column and synthetic raw water which contains As (Ⅲ),Fe(Ⅱ) and Mn(Ⅱ) in the concentration of 30-200 μg/L,0. 5-1. 5 mg/L and 0. 6-2. 0 mg/L,respec-tively. The results indicate that the removal efficiency of biofilter on As (Ⅲ) can be over 95%,and it is not af-fected by As with the concentration below 200 μg/L in the raw water. The capability of arsenic removal of the biofilter is found to concentrate in the filter bed with deepness of 0-660 mm after measuring the samples from different sampling places. And As (Ⅲ)and Fe(Ⅱ) will exceed the standards for a short time after back-washing.