Theoretical analysis of heat and mass transfer is developed for the unsteady evaporation of a moving droplet which is suddenly placed into the gas circumstances. A mathematical model is proposed for the unsteady evaporation process of a droplet at the first moment and is solved by numerical method. The results indicate that the evaporation rate is very high for a moving droplet suddenly placed into the gas circumstance at the initial moment, which can make the temperature of droplet surface decline. The scope of the temperature decline is concerned with the initial temperature of the droplet, gas circumstances, and the initial velocity of the moving droplet as well.