The invention relates to an automatic water adding device, in particular to an automatic water adding device for a water trough for animal husbandry. The automatic water adding device for the water trough for animal husbandry can replenish water resources in time, livestock can drink water by itself, time and labor are saved in the water adding process, and water adding efficiency is high. In order to achieve the technical aim, the automatic water adding device for the water trough for animal husbandry comprises a left side plate, a first connecting rod, a guide plate, a spring, the water trough, a pressing block, a button, a first sliding rail, a first sliding block, a first rack, a fulcrum rod, a right side plate, an alarm, a second sliding rail, a second sliding block, a second rack, a gear, a second connecting rod, a water tank, a first water outflow pipe and a first valve. Based on the gear and rack principle, the opening degree of the first valve is controlled through the water amount in the water trough, the flow of water is controlled accordingly, and the situation that water overflows when excessive, and growth of the livestock is affected when too little water exists is avoided.