A dehumidifier has a main body and a main tank. The main body has a secondary tank, a water pump, and a controlling assembly electrically connected to the water pump and having a starting position, a stopping position, and a float. The float is located in the secondary tank and is capable of moving with a water level inside the secondary tank. The controlling assembly controls the water pump to start or to stop according to the relative position between the float and the starting position or the stopping position. The main tank is detachably mounted on the main body, is located in a lateral side of the main body, and communicates with the water pump. With the controlling assembly, the water pump can be controlled to start only when necessary, thereby saving energy and reducing noises caused by prolonged running.