Abstract: To reveal the mechanism of the flashing/freezing process of a liquid droplet in vacuum, a new model of heat and mass transfer in the flashing/freezing process was established using a quasi-equilibrium process of mass and heat transfer. Based on the numerical results of the variation of droplet's size and temperature, influences of four factors, namely ambient pressure, ambient temperature, initial radius and temperature of the droplet, on the variation of pre-cooling time in the flashing sub-process, freezing time in the fast freezing sub-process and the final temperature of the process were analyzed in detail. Compared with the experimental data, the present model can predict the basic features of the process with a good reliability. The ambient pressure is the dominating factor determining the major features and the final temperature of the process. The initial temperature affects the flashing rate, while initial radius influences the pre-cooling time. The influence of the ambient temperature is too slight that can be neglectable.