A propulsion device of following kinetic energy (KE) penetrator of tandem warheads due to shear ring mode is designed. An interior ballistics model is set up with the hypothesis of instantaneous shearing action. The influence of different factors on the interior ballistic characteristics of propulsion device is discussed. The relations between the initial velocity of the following KE penetrator and chamber pressure with time and travel are obtained. The theoretical model is verified through experimental results and can be used as a reference for tandem warheads design.