The invention relates to the technical field of water supply and drainage of railway systems, in particular to a liquid level detection device of a sewage tank in a water supply and drainage sanitarysystem of a railway motor car, a passenger car, a locomotive and the like and application thereof. The device comprises a cavity structure shell consisting of an upper sealing cover and a cavity, wherein an upper end surface of the cavity is connected with the upper sealing cover, and a through hole is formed in the upper sealing cover; a first electrode and a second electrode, wherein one end ofeach of the first electrode and the second electrode extends into an ascending cavity after penetrating through the through hole on the upper sealing cover; an electrode fixing device arranged on theparts, located outside the external of the shell, of the first electrode and the second electrode, wherein the electrodes are supported on the upper sealing cover and can move in the through hole; a wire sliding nozzle, wherein the wire sliding nozzle is of a conical structure with an unsealed bottom surface, a hole is formed in the conical tip part of the wire sliding nozzle, and the bottom surface is connected with a lower end surface of the cavity; and an electrode on-off control mechanism located in the cavity, wherein the other end of a flexible rope is connected with a floating ball. Thedefect of inaccurate liquid level signal caused by the corrosion and dirt attachment on a liquid level device by the sewage device is overcome by utilizing a floating ball floating principle and theelectrode liquid level principle.