The utility model discloses an automatic dehumidifier of hidden cable pit belongs to electric power protection technical field. The utility model discloses a both ends open -ended cover body, the lower extreme and the cable pit intercommunication of the cover body, the upper end of the cover body is equipped with rain -proof cap, the fan is installed to the lateral wall of the cover body, an on the lateral wall that is used for the humidity transducer of the interior humidity of detection streamer ditch to install at cover body lower extreme, install the control module who starts with the humidity transducer response on the motor of fan, the humidity signal transmission that humidity transducer will detect in the cable pit gives control module, when humidity in the cable pit is higher than the predetermined value, the motor of control module control fan starts, to carrying out the ventilation dehumidification in the cable pit. The utility model discloses can improve the ventilation effect in the cable pit, reduce the interior humidity of ditch, avoid the cable in the cable pit to receive the damage, reduce the potential safety hazard of electric power operation.