In order to investigate the effect of die wall thickness on morphologies of defect band, a stepped mold with a wall thickness of 5 mm, 4 mm, 3 mm, 2 mm, and 1 mm was designed to carry out high pressure die casting experiments with AlSi10MgMn alloy. For castings with wall thickness of 2-4 mm, the ratio of the mean defect band width (w) and mean grain size (d) in the defect band (w/d) ranges 7-18, while it increases to 24.47 for the 5 mm-thick casting. This difference is related with the filling speed and the distribution of externally solidified crystals (ESCs). The mold flow analysis indicates that the filling speed decreases from 25.41 m·s^-1 to 11.07 m·s^-1 when wall thickness increases from 2 mm to 5 mm. Due to the decreasing filling speed along the wall thickness, ESCs gradually diffuse from the center to the defect band, which keep the shear strength in the defect band at a high-level during filling. Meanwhile, the shear strength generated during the filling also decreases as the shear rate drops. Finally, the defect bands in the 5 mm-thick region become widen and indistinct, and the porosity is as high as 5.25%.