This paper presents a calculation of the vibrational spectra of hydrogen atoms and the probability distributions for protons along the hydrogen bonds to study the hydrogen-bond symmetrization in ice by performing ab initio molecular dynamics simulations. The disappearance of the translational peaks of ice VIII is observed, which is interpreted as a transition into disordered ice VII. Two peaks of the translational modes related to the hydrogen bond are observed again in the megabar range; meanwhile, a new band becomes a definite peak, which is assigned to the IR-active translational vibrational mode of ice X, both strongly indicating that a phase transition to ice X has occurred. Moreover, the proton probability distributions that can locate the proton position provide an intuitive way to understand the symmetrization process. The results give the reasonable interpretations of the conclusions obtained from the vibrational spectra of hydrogen atoms.