The vaporization of a single water drop is studied in the paper theoretically. It gives help to the study about drop group. Vaporization-time raises with increase of drop diameter, or with decrease of drop speed, or with decrease of drop temperature. It is helpful to coding gas that vaporization of drop can absorb a lot of heat. The vaporization-time can be controlled by adjusting the parameters, such as drop diameter, flux and so on. Further more, these can give help to estimate the height of cooling tower, and then to decrease the cost.