In order to study the influence of transition zone length on stretching bending, in this paper, typical asymmetric aluminum profile used in high-speed train was selected as the research object, and stretching bending research of four different transition zone length was carried out. Numerical simulation was used to analyze and compare stress and strain distribution, shape error, springback, cross-section distortion and spatial twist, and the forming defects index was defined to characterize the integrated forming effects. The results reveal that an increment of the post-stretching elongation leads to small shape error and springback but large cross-sectional distortion and spatial twist. The forming defects index illustrates that a 200 mm transition zone leads to the least forming defects for stretching bending. The 0.618 method was used to optimize transition zone length. Experimental validation was also conducted on stretch-wrap bending machine to show the consistence with the numerical analysis.