This work has shown a method for simulating the motion of beads in a stirred mill based on the discrete element method (DEM) under wet grinding of gibbsite powder, in correlation with the experimental work. The beads' motion was compared with the experimental data monitored by visualization. The movement obtained from the simulation agrees with the experimented one. The specific impact energy of beads was calculated on the base of the simulation. The role of the pin configuration, especially, the number of pins of agitator was investigated on the beads motion and the specific impact energy. As a result, the number of pins is an important factor governing milling performance and its optimization is determined under the condition.