Abstract: It is well known that bending of the thin-walled tubes during continuous straightening causes ovalization in the tube cross-section, so it must be flattened to restrore roundness subsequently. Therefore, analyzing the deformation process and mechanism of flattening for setting round, the normal circumferential strain component is derived based on the elastic-plastic theory and relevant hypothesis of thin-walled members, then the circumferential bending-moment is obtained, and finally the mathematical model of the flattening intermesh for setting round is proposed using the classic unloading rule. In order to certify whether it is correct, we have done some dynamic simulations by FEA, and the results have shown that the model is correct and suitable, it was also found that the twice the maximum initial radial displacement can be used as a simplified estimate for the flattening intermesh. This paper can provide the basis for optimizing straightening technical parameters and completing the theory of thin-walled tube straightening.