Abstract: Based on the variational principle of unknown boundaries formed in the forming process of an ideal elastoplastic straight beam of rectangular section, this paper carries out the variational calculation of the potential energy to derive the deflection curve equation of a cantilever beam bending mode, and to obtain the solution to the bending of the proflling forming straight beam. According to the calculation results, the deflection equations of the proflling forming straight beam can be derived with the application of the variational principles of potential energy and complementary energy.