Abstract: In order to understand the formation and development law of the cavity with the small caliber rifle bullet penetrating the gelatin, an experiment was carried out, taking 4℃/10% concentration ratio, size of 300mm×300mm×300mm gelatin as target, and the high speed photography for measurement. Three 5.56mm SS109 bullet's high speed photography and pressure curves of penetrating gelatin at 100 meters range were presented. Through processing the data and comparatively analyzing with the data of 7.62mm rifle bullet, results show the following conclusions:the max diameter of temporary cavity is within 200~270mm, the pulse cycle is about 11μs; the diameter of temporary cavity is attenuating as half sine wave curve with the penetration time; the shape and size of the killing element influence remarkably on the shape of the temporary cavity, thereby can relate to the lethal power of the ammunition.