Straight metal pipes are widely used in petroleum exploration and transmission equipment systems. Straightening is one of effective methods for ensuring or repairing the bending during straight metal pipe rolling, heat treatment, assembly and operational application. However, the flatten phenomenon of straight metal pipe during straightening process is unavoidable. Therefore, it is significant to study the flatten phenomenon of straight metal pipe in straightening process. Based on the theory of elastoplastic deformation, the flatten phenomenon of straight metal pipe is analyzed. Further, the straightening stroke model of straight metal pipe for guiding the straighten operation is established. Afterwards, the theoretical straightening stroke model is analyzed on the basis of simulation. Finally, the experimental system was built up. A series of related experiments were carried out to verify the rationality of the model.