A dehumidifier with multistage draining has a container that collects and stores condensed water from a dehumidifying assembly, a float mounted in the container, a float actuator connected to the float and a drain motor connected to the float actuator and an attachment actuator having a switch. When an external pipe is connected to the attachment connector to switch on the switch and the condensed water in the container gradually raises the float to switch on the float actuator, the drain motor is switched on to pump the condensed water out of the container. The dehumidifier automatically drains condensed water in the container according to the quantity of the condensed water, does not have to be cleared frequently and is convenient.