The Raoul effect makes a great influence on the accuracy of measuring dew point of gas using a hygrometer. When estimating the Raoul effect by measuring condensate freezing point, we found drops of alcohol on the cold mirror for blowing and heating evaporation residue alcohol to clean the surface can greatly reduce the Raoul effect. It can reduce the Raoul effect’s error of dew point temperature to less than 0.1℃.