In order to study the basic properties of cotton straw biochar and its adsorption effect of arsenic, the spectral properties of modified cotton straw biochar with FeCl3·6H2O was explored using XRD, FT-IR and SEM analysis, and its adsorption capability of arsenic(As) was investigated. The results show that pH value, specific surface area, contents of C, N, H elements and C/N ratio of the modified cotton straw biochar modified with FeCl3·6H2O significantly decreased with the increase of Fe content, whereas ash content and element O, H/C, O/C and (N+O)/C contents increased significantly with the increase of Fe content, as well as Fe2O3 and Fe3O4 contents on the surface of the modified cotton straw biochar. The balance time of arsenic adsorption was greatly shortened after the modification, and its adsorption rate was higher than 73.4%, far higher than that of the unmodified biochar (44.7%), with the adsorption capacity of 7.63 mg/g, which is much greater that of the unmodified biochar (4.33 mg/g). In addition, the adsorption rate and capacity significantly increased with the increase of Fe content. The adsorption mechanism behind these phenomena is that the electrostatic adsorption capacity of the modified biochar, ion exchange and the reduction of Fe3+ in the aqueous solution reduce the As content, which in turn reach the goal of As removal in the aqueous solution.