A compartmented rhizo-bag culture system was used to study the effects of exogenous phosphorus on arsenic fractions in the rhizosphere and their bioavailability to rice (Oryza sativa L.) and wheat (Triticum aestivum L.) seedlings growing in arsenic-contaminated soil from Chenzhou in Hunan province. The proportions of arsenic fractions in rhizosphere and non-rhizosphere soils for rice and wheat growth followed the order: well-crystallized hydrous oxides of Fe and Al (45%-52%) > amorphous and poorly-crystalline hydrous oxides of Fe and Al (26%-34%) > specifically-sorbed (12%-14%) > residual phases (4%-7%) > non-specifically sorbed (0.09%-0.25%). P100（added P concentration 100 mg·kg-1） increased the shoot biomasses of rice when compared to P0（no added P）treatment. As for the wheat, P100 not only significantly increased the shoot biomasses but also inhibited the translocation of As from root to shoot. The results also showed that the SPU(Specific P Uptake) and SAU(Specific As Uptake)of rice as well as its TF-As(Translocation factor) and TF-P were higher than those of wheat at all P treatments. As a result, the best method would be to plant wheat in low-medium arsenic contaminated soils. During the period of rice growing, P50 treatment would be the best choice for decreasing arsenic risks in rice plants.