Abstract：The objective of this study was to investigate the effect of iron (FeCl3 · 6H2O) modified biochar on the migration capacity and morphology of As in soil. The biochar was made from cotton straw. The modified biochar was prepared by 20:1 biochar and FeCl3·6H2O. The pH value of biochar was 11.4 and that of the modified biochar was 8.5. Soil at 0-20 cm layer of farmland was collected for column experiments. The column was 50 cm in height. The 0-20 cm of soil column was the mixture of biochar or modified biochar and soil at the ratio of 0, 1%, 2%, 4% and 8%, respectively. The bulk density of soil column was consistent with that in the field. The column was irrigated for 20 times. The irrigation amount was 200 mL for each time. The filtrates and soil samples were collected to determine the As content and forms. The results showed that the total As content values in filtrates from soil columns with 1% - 8% biochar were gradually decreased with irrigation times. After two times of irrigation, the As content was lower than 10 µg/L (national security drinking water standard in China). However, the values of As content in all the filtrates from soil columns with 1%-8% iron modified biochar were lower than 5 µg/L, which was much lower than the standard. The content of As in the soil decreased with the increase of addition amount of biochar and iron modified biochar. It also decreased with increasing soil depth. The content of As in the first layer of soil were the highest. In the biochar treatments, the proportion of As in the first layer accounted for 45.46% (soil with 1% biochar addition), 48.41% (soil with 2% biochar addition), 48.88% (soil with 4% biochar addition) and 51.92% (soil with 8% biochar addition) of the total, respectively. In the iron modified biochar treatments, the proportion of As in the first layer accounted for 51.71% (soil with 1% modified biochar addition), 51.99% (soil with 2% modified biochar addition), 54.46% (soil with 4% modified biochar addition) and 60.26% (soil with 8% modified biochar addition) of the total, respectively. The content of modified biochar in the first layer was 1.42-1.66 times of that in soils without modified biochar addition. Thus, the As migration in soil could be prevented by adding biochar in 0-20 cm layer, and the inhibition role of the iron modified biochar was stronger than that of biochar. In the soil without biochar, the As was present in six forms including residue arsenic (R-As), calcium arsenic (Ca-As), water arsenic (W-As), exchange arsenic (E-As), aluminum arsenic (Al-As) and iron arsenic (Fe-As). After adding biochar, the As still occurred in the six forms but the content of W-As decreased while the content of R-As increased in the layer less than four, and the R-As and Al-As were the two main forms in the fifth and sixth layers. The soil with iron modified biochar had a small amount of W-As and E-As, and R-As, Ca-As and Al-As were mainly present in the first and second layers. Therefore, adding iron modified biochar into soil greatly decreased the content of W-As and increased the content of R-As. The iron modified biochar can promote transformation of As forms into R-As, which will reduce the risk of As to pollute soil.