The migration mechanism of the target material in the millisecond laser drilling process directly affects the morphology of the hole. Therefore, the study of it was of great significance for the optimization of laser parameters. The migration of the material includes the migration of off-target gasification products, sputtered molten materials and non-off-target molten materials, and the molten materials was re-solidified to form the recast layer. For the off-target material, the high-speed camera was used to observe the movement law; For the recast layer, the metallurgical microscope was used to observe its morphology; Finally, the mass loss was measured with an analytical balance. The results show that the phase explosion threshold of aluminum alloy is 1 047.24 J/cm². When the energy density is below the phase explosion threshold, the off-target material mainly migrates in the form of vaporization, and form circumferential and radial cracks. When the energy density is above the phase explosion threshold, the molten material is washed out of the ablation hole, and there is almost no recast layer inside the hole. The quality of gasification loss is very small. The mass loss mainly comes from the melt splash driven by the phase explosion. The phase explosion increases the single-pulse ablation quality by 10.7 times. Both the phase explosion and the decrease of laser intensity cause the vaporization rate of the target material to decrease.