The invention provides a powder automatic dispensing device and a washing machine. The powder automatic dispensing device comprises a casing and a screw. The screw is rotatably arranged in the cavity of the casing and is connected with a driving device. The screw is a variable diameter screw, and is divided into a large diameter section, a transition section and a small diameter section. The transition section is arranged between the large diameter section and the small diameter section of the screw. The thread diameter of the transition section gradually becomes from large to small, and is continuous with the thread of the large diameter section. The threads of the large diameter section and the transition section of the screw are in single helix structure. The small diameter section of the screw is in double helix structure. A discharge pipe which is communicated with the cavity of the casing and is horizontally placed is arranged at the bottom of the casing. The small diameter section of the screw runs through the discharge pipe, and the outer diameter of the small diameter section of the screw matches the inner diameter of the discharge pipe. The end part, which is away from the casing, of the discharge pipe is closed, and a discharge port is arranged in the pipe wall at the bottom of the discharge pipe. Through the delivery, compression and constant volume of materials, the dispensing of powder materials is accurate. Through the powder automatic dispensing device on the washing machine, powder is accurately dispensed.