The invention discloses an automatic livestock drinking trough and a method thereof. The drinking trough comprises a mechanical part and an electrical part. The mechanical part comprises a water tank, a floater, a lever, a rotary support, a connecting rod, a cross rod with a hole, a fixing support, an armature, a magnet, a spring, a water supply pipe, and pin rolls. The electrical part comprises a DC power supply, a plug, a relay, and a solenoid valve for water. The rotary support is rotatably connected with the lever through the first pin roll. One end of the lever is rotatably connected with the connecting rod through the second pin roll. The connecting rod is fixed with the floater. The other end of the lever is fixedly provided with the magnet. The magnet is provided with conductive electrode plates. The fixing support is fixed with the spring. The lower end of the spring is fixedly provided with the armature. The armature is electrically connected with the relay, the DC power supply, and the conductive electrode plates. The solenoid valve for water is electrically connected with the plug. The relay controls the solenoid valve for water so as to control on-off of the water supply pipe. The water tank is provided with the cross rod with a hole to fix positions of the floater. The drinking trough is simple in structure, and can automatically supplement water of the livestock drinking trough according to water level.