Disclosed by the present invention is an automatic additive-feeding device, comprising a liquid storage box for storing a liquid additive, a measuring chamber in communication with the liquid storage box, an additive pumping unit for pumping additives in the liquid storage box into the measuring chamber, and a flushing water path in communication with the measuring chamber, wherein the measuring chamber may measure the amount of additive entering the chamber; the flushing water path introduces inlet water into the measuring chamber; and then the additive pumped into the measuring chamber by the additive pumping unit is discharged, and the measuring chamber is flushed; the measuring chamber may directly communicate with the liquid storage box or indirectly communicate with the liquid storage box, and a one-way passage which flows from the liquid storage box to the measuring chamber is provided between the measuring chamber and the liquid storage box. According to the present invention, a simple one-way passage is provided between the liquid storage box and the measuring chamber, so as to be able to prevent flushing water from being guided into the liquid storage box in the process of flushing the measuring chamber. The structure is simple, the control time sequence of automatic feeding is simplified, and costs are also saved.