Abstract: The unsteady vertical cavity produced by the underwater cavitator moving rapidly towards the water surface is investigated theoretically. The mathematical model of the unsteady vertical cavity under water is presented; the formula of the vertical cavity length influenced by the gravity and water surface is derived, and the life span of cavity shedding from exit-water cavitator is also obtained from the formula. For the underwater body with cavitator, the acting position in the body of the high-pressure produced by the collapsed cavity is also calculated by the formula; moreover, the condition and criterion of keeping the underwater body from collapse high-pressure is presented.