The invention discloses an automatic detergent feeding device for a washing machine and a control method of the automatic detergent feeding device. The device comprises a water injection trough, a distributor box arranged in the water injection trough, a constant-volume box arranged at the back part of the water injection trough, a suction mechanism for leading a detergent into the constant-volume box, storage boxes, a flushing water inlet and a flushing pipe for discharging a liquid in the constant-volume box, wherein the storage boxes and the flushing water inlet communicate with the constant-volume box, the storage boxes are arranged in the distributor box, and check valves, which are opened when a distributor is opened and are closed to prevent a liquid from flowing out when the distributor is closed, are arranged between the storage box and the constant-volume box. A main water inlet and a main water inflow pipe are integrated on the water injection trough, the main water inlet communicates with the distributor box through the main water inflow pipe, a detergent feeding chamber is formed in the distributor box, and the main water inflow pipe leads to the detergent feeding chamber. The detergent is sucked into the constant-volume box by the suction mechanism, is then diluted and flushed to a washing cylinder by inflow water from the flushing water inlet. The device has the characteristics of being compact in structure and facilitating modular mounting and utilization of a user.