The flanging on both sides of wheel cowling tail of front fender is stretch flanging. An asymmetric flanging model is built according to the asymmetric twist defect in flanging. The strain distribution of the flanging regions, which is analyzed through finite element method (FEM), is in reasonable agreement with related theories. The result shows that the built model is appropriate to investigate the flanging twist defect of the fender. The accuracy of the model is proved by the comparison of the simulation result with the practical production. Also it shows that the severity of the distortion mainly relies on the asymmetry of the fender and some suggestions are proposed on distortion control through the analysis of the factors influencing laws on the distortion angle, which is define to measure the twist, like radius ratio of the two sides of the fender, flanging height, the clearance between the die and punch of the flanging tools, the fillet radius of the flanging die.