In order to research the distributed rule of the fragment field, the numerical simulation of the prefabricated-fragment warheads exploding was realized by using the Arbitrary Lagrangian-Eulerian (ALE) algorithm in this paper. The finite-element model of disperse fragments for monolayer warhead was established. The diffuseness of fragments and the pressure distribution field were calculated, and the distributed function of the fragment density was deduced. The simulation result agreed comparatively with the results of experiment. It shows that the rationality and practicability of the simulation with the ALE algorithm is validated for simulating the prefabricated-fragment warheads.