For L-section aluminum profile part in a rail vehicle, the causes producing the section distortion defects in stretch-bending process were analyzed by the numerical simulation, and the influences of the bending die structure parameters on the section deformation were studied. Then, the corresponding defect controlling methods were proposed and verified by tests. Results show that the general section distortion defects are the vertical wall obliquing and the horizontal plate collapsing. The former is caused by the unreasonable holder gap, which can be controlled by optimizing the holder gap, while the later is mainly induced by the shrinkage of the vertical wall, which can be controlled by decreasing the depth of the die supporting surface based on the shrinkage degree of the vertical wall.