Abstract: Damp atmosphere in explosion-proof enclosure of explosion-proof water-cooling frequency converter could lead to condensation during hot and cold variation, worse still, the condensed water may cause short circuit and destroy the converter. The condensation mechanism was qualitatively analyzed, and the relationship model was established about air temperature, air humidity and dew-point temperature, which was applied to determine whether there was condensation in the cavity. The correctness of the relationship model was verified by simulation test. The influence of flow and temperature of cooling water on condensation was analyzed, and a conclusion was gotten that the bigger difference between atmosphere temperature and dew-point temperature, the more serious condensation. It was pointed out that condensation in the explosion-proof enclosure can be controlled by means of cooling or heating.