To obtain the cross-sectional distortion of thin-walled rectangular tube in rotary draw bending process accurately, a new method to exactly describe the distortion has been proposed. The coupling between the convex of the tube side walls and the concave of the tube flanges is considered in the distortion modeling. Combined with the experiments, the cross-sectional distortion obtained using the new distortion model are compared with the previous ones obtained without considering the convex based on numerical simulations. The results show that cross-sectional distortion obtained by current model is closer to reality than the previous ones. The distortion by current model is more safety in the practical bending process.