Abstract: The common shape defect of warping deformation, which is caused by the uneven vertical elongation of strips in the thickness direction, was studied by applying symplectic elasticity method. The relationship between the warping value and the distribution of vertical elongation was obtained through establishing and solving a mechanical model of warping deformation. Based on the correctness of the mechanical model and calculated results which were verified by general finite element method, the influence laws of factors on warping deformation were gotten by deduction and calculation. It is shown that the warping value increases linearly with the increasing elongation difference between the top and bottom surfaces and the increasing ratio of the transverse to vertical elongation difference, decreases with increasing strip thickness, but increases quadratically with increasing strip width. Finally, the phenomenon of C-warping converting to L-warping was studied by using the symplectic elasticity model of warping deformation, a mechanical model of C-warping converting to L-warping was established, and the formula for this converting was obtained, which was verified by field experiment.