Flanging U-shaped piece is the typical auto-body part, such as tailor-welded front rail inner panel, whereas, large springback amount is a critical challenge in sheet metal forming process, which size and shape accuracy affect the quality of the following assembly process. Firstly, a new form of variable blank holder force (BHF) was proposed in this paper, the springback problem was analyzed by numerical method based on the constant BHF of 90 t, and the contours of Von-Mises stress and springback amount were calculated by the dieless method. Secondly, variable BHF with changes in position and punch stroke was designed and used to control springback. Finally, orthogonal experimental and range analysis method were used to optimize the variable BHF parameters. In addition, the springback angles of sidewall and flange under 16 different combinations were calculated. The results show: 1) The thinner and weaker sheet near the weld-line position prone to fracture; 2) The maximum stress after springback decreases by 14.2%, therefore, the release of residual stress is a main reason for curl springback in the sidewall and flange; 3) The maximum springback amount of 2.68 mm is located at the opening zone of flange, where the sheet material is in the free state; 4) The maximum thinning rate under the variable BHF is less than that of the constant BHF, which indicates that the variable BHF control method improves the forming limit of sheet metal; 5) The maximum springback angle of sidewall and flange decreases by 33.05% and 48.64% respectively.