Abstract: The cavity closes on the body surface with a re-entry jet at its rear when the body moves with a high speed and induces a cavity in the fluid. The re-entry jet at the closure position of the cavity causes the overpressure on the body. The closure position moves with a considerable speed when the cavity is in the gravity field or when the body velocity or the pressure in the cavity varies. The unsymmetrical distribution of the moving closure position with overpressure arouses significant effect on the motion stability of the body. As far as we know, no theoretical researches about overpressure on the moving closure position owing to the unsteady of the cavity have been reported in literature. In this paper, based on the theory of the potential flow, the position of maximum pressure in closure region of unsteady cavity in gravity field is investigated, and the theoretical formula of maximum pressure is presented, which reveals the relation between the maximum pressure and the developing speed of the cavity. Two special fluid characters of maximum pressure are proved. Finally, a verification experiment is designed and performed for measuring the maximum pressure at the closure position, and the comparison is finally performed between the results of the theoretical formulation and the experiment, which confirms the theoretical approach in this study.