：In the roller-type straightening process and setting round process, the initial curvature is unified by means of the reciprocating bending. The deformation law of reciprocating bending is the theoretical basis for determining the process parameters. In the reciprocating bending process, a set of symbol system is established, and the bending curvature and moment are both vector. The small curvature plane bending springback equation and strain superposition principle are the theoretical basis. The initial equivalent strain and equivalent strain are introduced. Considering the deformation hardening, Baushinger effect and cyclic softening, the reciprocating bending springback process is analyzed by using the graphic method. It shows that the reciprocating bending eliminates the difference of initial curvatures and makes the curvatures be in the same direction and have same value. The uniform curvature theorem by reciprocating bending is presented. The bending test device is designed. The different material plates that has different initial shape are used to carry out the reciprocating bend test. The variation of residual curvature and the deviation of fitting arc are discussed. The correctness of the uniform curvature theorem is proved. It lays the theoretical foundation for the development of process plan and control strategy in the roller-type straightening and setting round process.