Molecular dynamics calculations and several experiments were performed to investigate the physical properties of the titanylphthalocyanine Y-form crystal. The proposed crystal structure by Oka et al. is confirmed as stable at room temperature. It is suggested that the dependence of the photosensitivity on the humidity is caused by the water molecules on the surface of the Y-form. A mechanism for the phase transition from the Y- to A-form is proposed. At about 250 °C the water molecules observed in the Y-form in this study begin to jump from one space to another in the crystal through the channels connecting them and as a result are expelled out of the crystal. After the water molecules go out of the internal spaces, the phase transition to the A-form occurs by changing the molecular arrangement to shrink these spaces. It has been proposed that the high photosensitivity of the Y-form is due to its molecular arrangement in terms of the transition moment. Since the photosensitivity is raised by the water molecules on the surface, these observed inside of the Y-form might also contribute to increase the photosensitivity.