In order to evaluate the lethality and injury radius of the stun grenade fragments, and to reduce its average specific kinetic energy, the calculation model of the average specific kinetic energy of fragments was established, based on the Monte Carlo subdivision projection simulation method. With co-simulation method of LS-DYNA and self-programming, the calculation models of average specific kinetic energy of natural fragments and semi-prefabricated fragments of stun grenade were simulated and compared separately. The mass distribution, initial velocity, vertical target distribution, average specific kinetic energy threshold and safety radius of the whole fragments in the whole-time domain were obtained. The results show that the mass distribution and initial velocity threshold of semi-prefabricated fragments are lower and more concentrated than that of the natural fragments under the same charge parameters; within the range of less than 2.5 m, the average specific kinetic energy threshold of semi-prefabricated fragments is lower than that of the natural fragments; within the range of more than 2.5 m, the average specific kinetic energy threshold of semi-prefabricated fragments is higher than that of the natural fragments. Compared with the natural fragments, the semi-prefabricated fragments can significantly reduce the lethality radius, but cannot significantly improve the safety radius.