Abstract: The long-term operation of ring main unit (RMU) in humid condensation environment will lead to the decrease of insulation strength of equipment, which will result in discharge and breakdown faults, and further shorten the service life of equipment. Therefore, it is very necessary to ensure a good operation environment for RMU. Semiconductor condensation dehumidification is an emerging dehumidification technology that controls the humidity in the cabinet by condensing moisture and discharging it out of the cabinet. In this paper, the COMSOL simulation software is used to simulate semiconductor condensation dehumidification. The air flow field, heat field and water transport in the dehumidifier are coupled to study the distribution changes of flow field, temperature and relative humidity after the installation of the dehumidifier, as well as the influence of dehumidifier installation position on dehumidification effect. The results show that the temperature inside the RMU changes little after the installation of the dehumidifier, the relative humidity can be reduced from 90% to 50% in about 11h, and the dehumidification effect is the best when the dehumidifier is installed at the bottom of the cabinet. The results of the simulation are verified by experiments. In this paper the influence of installation position of dehumidifier on temperature and humidity distribution and dehumidification effect in RMU is given, which can provide a reference for installation of dehumidifier in RMU.