Abstract: This paper systemtically analyses the pattern of ventilation through the vent of fruit underground storage cavse; and establishes the mathematical model of the natural ventilation. A new conception——the effective height of the vent is provided according to the model, and further more, a formula for culculating the optimum height of the vent is developed. The results show that the optimum height of the vent is far lower than that has been used in practice, and the rate of air exchange through the vent may increase one time or more.