The protective shell plays an important role in preventing the aircraft from harmful environment impact. When it becomes an irregular body falling into the water, its motion is hard to predict. In this paper, firstly, a semi-cylindrical shell is taken as an example. Subsequently, the dynamics modeling of the irregular body is established, and the dynamics equations and the kinematics equations describing the underwater falling motion of the irregular body are presented, respectively. Finally, simulation is carried out to predict the falling process and analyze the typical trajectory of the underwater falling motion of the irregular body.