Rolled piece deformation is analyzed by FM-BEM in straightening process. Deformation rule of plastic deformation area is analyzed in different straightening force, especially the area of the nodes in neutral axis. Through numerical analysis, it shows that plastic deformation of neutral axis nodes applied 250 kN straightening force is smaller than adjacent nodes. With straightening force increasing, this difference disappeared and formed a relatively flat area in 500 kN. The results obtained by FM-BEM demonstrate that neutral layer has some offset under straightening force. Compression bending amount has relationship with the neutral layer offset and it is one of reasons that compression bending amount is inaccurate. These conclusions are not obtained by FEM.