The invention provides a liquid supplementing device. The liquid supplementing device comprises a first guide pipe, a second guide pipe, a first liquid supplementing container, a second liquid supplementing container and a controller. A first vertical pipe of the first guide pipe communicates with the bottom of the first liquid supplementing container. A transverse pipe of the first guide pipe communicates with the bottom of the second liquid supplementing container through the second guide pipe. A second vertical pipe of the first guide pipe communicates with the top of equipment to be supplemented with liquid, wherein the equipment is located below the first liquid supplementing container and the second liquid supplementing container. A valve is arranged on the transverse pipe of the first guide pipe or the second guide pipe and is connected with the first end of the controller. The controller is used for controlling opening or closing of the valve. The first liquid supplementing container is used for supplementing liquid to the equipment to be supplemented with liquid when the valve is closed. The area of the cross section of the first liquid supplementing container is smaller than that of the cross section of the second liquid supplementing container. The problem that as displaying of the liquid level changes is not obvious in the prior art, it is difficult for workers to find slight reduction of liquid inside a water tank is solved.