We present the study of the interaction of an intense circularly polarized pulse with a solid target with one-dimensional (1D) particle-in-cell (PIC) simulation. The evolvement of ion motion with time is explained by a purely kinetic description and by the theory of electrostatic shock in collisionless plasmas. Especially the formation of the stable profile with a "double-flat-top" in ion phase space is explained and validated visually. Assuming the initial state, we find that the ion distribution in the phase space agrees qualitatively with the PIC simulation results by using the particle-tracing approach.