The invention discloses a water treating device with float type feeding and drainage mechanisms. The device comprises a filter tank, a filter material layer and a back washing pipeline, a water purifying outlet is formed in a tank wall below the filter material layer, a water inlet and outlet pipe is vertically arranged in the filter tank, and the lower end of the water inlet and outlet pipe is connected with a muddy water drainage pipe and a water feeding pipe for water to be filtered outside the filter tank through a pipe; the upper end of the water inlet and outlet pipe is sleeved with a float capable of suspending in water, the upper end of the float is open to form a water inlet and outlet, the outer side wall of the water inlet and outlet pipe and the inner side wall of the float arein sliding sealed connection, the float can float up and down along the water inlet and outlet pipe along with changes of the water level inside the filter tank, the height of the water inlet and outlet changes along with changes of the water level, in the back washing drainage process, the device is matched with the liquid level of the filter chamber all the time, the phenomenon that filter materials are lost along with water is avoided, the back washing efficiency is higher, the back washing frequency can be lowered, the back washing time is shortened, and the using efficiency of the filtertank is improved.