In order to evaluate kill efficiency for small arms effectively, kill efficiency for small arms was theoretically evaluated and calculated. Take the terminal ballistics theory and wound ballistics theory as reference, The kinetic model of Spherical fragment and bullet in gelatin was established, then the kinetic model was programmed by using the Runge-Kutta method, the penetration velocity and the penetration depth verse time for damage element were calculated by using this program, the transferred energy of damage element in gelatin was predicted. Then the transferred energy was applied to the evaluation models of kill efficiency for small arms based on the energy-transferring, so the kill efficiency of damage element could be estimated quickly. It was demonstrated by the instance that this method is simple and convenient, and could provide referrence for study, design and demonstration of small arms.