Modeling the penetration process in soft tissue simulant is the basis for evaluating the damaging efficiency of projectiles. To investigate the penetration dynamics of fragments in gelatin, this paper presented a model describing both the penetration resistance and the temporary cavity dynamics. The modeling process was set up using the law of energy conversion and conservation. The equations for the movement of the penetrator and the movement of the target medium are closely combined through sharing the same set of parameters. Penetration experiments were conducted using fragments of two different shapes: cylinder and rhombus. By comparing with the experimental results, the parameters in the present model were estimated and discussed thoroughly. The present model can predict the movement of the penetrator and the cavity expansion in the target with satisfactory accuracy.