The perforation mechanism of the high-strength armor by Armor Piercing Bullet (AP) was investigated by experiments. A theoretical model for the ballistic limit of the armor and residual velocity of the bullet including the effects of the strain rate and the thermal softening of the material was presented. The results show that the theoretical values have fine agreement with those of the experiments. The impact of failure criterion on the results was analyzed and the temperature rising of the shear band and the change rules of energy-dissipating of the target with the input velocities were discussed.