Abstract: In view of problem that mine-used frequency converter is easy to be damaged under situation of condensation, a humidity controller of mine-used frequency converter was designed. The humidity controller is composed of main controller, temperature and humidity sensor, condensation sensor and heating device. When inside of the frequency converter has condensation caused by big temperature difference of inside and outside of the frequency converter, the main controller makes the frequency converter to stop working after detecting blocking signal of the condensation sensor, meanwhile the main controller opens heating device to accelerate water evaporation in the frequency converter and sends alarming signal. When the main controller detects internal environment of the frequency converter is recovered, it cancels the alarming signal and makes the frequency converter to recover normal working. Actual test result shows that the humidity controller can ensure mine-used frequency converter work reliably under moist environment.