Abstract: The flattening machine made the aluminum strip perform repeatedly stretch bending deformation before cold rolling process. The flattening process has a vital influence on strip rolling process by improving plate shape and residual stress distribution of the strip in the same time. In this paper,the flattening process was researched by finite element method software Abaqus according to a five roll flattening machine. In order to describe behaviors and principles of the flattening process,a series of new definitions and descriptions were put forward. Based on numbers of simulation of different kinds of shape defects and systemic analysis,relations between the effect of flattening and different degrees of plastic deformation were discussed. The results show that the best flatness parameter is decided by the shape of itself.