Cross-sectional ovalization of thin-walled circular steel tube because of large plastic bending, also known as the Brazier effect, usually occurs during the initial stage of tube's continuous rotary straightening process. The amount of ovalization, defined as maximal cross section flattening, is an important technical parameter in tube's straightening process to control tube's bending deformation and prevent buckling. However, for the lack of special analytical model, the maximal section flattening was determined in accordance with the specified charts developed by experienced operators on the basis of experimental data; thus, it was inevitable that the localized buckling might occur during some actual straightening operations. New normal strain component formulas were derived based on the thin shell theory. Then, strain energy of thin-walled tube (per unit length) was obtained using the elastic-plastic theory. A rational model for predicting the maximal section flattening of the thin-walled circular steel tube under its straightening process was presented by the principle of minimum potential energy. The new model was validated by experiments and numerical simulations. The results show that the new model agrees well with the experiments and the numerical simulations with error of less than 10%. This new model was expected to find its potential application in thin-walled steel tube straightening machine design.