Abstract: No matter what the flatness of a strip is, flat or defective, at an annealing furnace entrance, the strip can not keep its original shape and a remarkable change of its shape can be seen at the annealing furnace exit. By investigating this phenomenon at the 2030 mm continuous annealing line which belonged to Baosteel, a finite element model of thermo-mechanical buckling deformation of strips in a continuous annealing furnace were established, and the mechanism of flatness changing and the contributing factors were researched by the finite element software ANSYS.