The invention discloses an automatic cotton feeding device for a cotton mixer. The automatic cotton feeding device for the cotton mixer comprises a cotton mixer body and automatic cotton feeding device bodies symmetrically arranged on two sides of a feed inlet of the cotton mixer body; each automatic cotton feeding device body comprises a cotton collecting box and a cotton conveying device, each cotton collecting box is arranged on the cotton mixer body, and each cotton conveying device is arranged on the bottom of the cotton collecting box; a motor is arranged on the top plate of each cottoncollecting box, the output end of each motor is connected with a stirring shaft, and a plurality of stirring rods are uniformly arranged on each stirring shaft; a through groove is formed in the bottom end of the front side plate of each cotton collecting box in the vertical direction, a sliding block moving up and down is inserted into each through groove, an electric rod is installed on the front side plate of each cotton collecting box on the lower part of the corresponding sliding block, and the extending end of each electric rod is connected with the corresponding sliding block; cotton blocking plates are symmetrically hinged to the inner walls of the left side plate and the right side plate of each cotton collecting box, a connecting rod is hinged to the front side surface of each cotton blocking plate, and the other end of each connecting rod is hinged to the corresponding sliding block. According to the invention, each cotton blocking plate is arranged in the corresponding cotton collecting box to control the speed at which a mixture falls from the cotton collecting boxes to the cotton conveying device, so that the cotton mixer is prevented from idling and stopping due to excessive feeding.