The utility model relates to a mixing drum of a mixer. The mixing drum can be used for effectively solving the problem that the material mixing effect is poor on the edge of a cavity of a conventional mixing drum. According to the technical scheme, the mixing drum comprises a drum body, wherein a sliding chute is formed in the wall of the upper end part of the drum body and incompletely penetrates through the wall of the drum body, a hinge is arranged in the sliding chute in a sliding manner, an arc-shaped plate is arranged on the hinge, a spring is arranged on the inner wall of the other end of the arc-shaped plate, a rotatable pull rod is arranged in the spring, one end of the pull rod is fixed on the inner wall of the arc-shaped plate, and the other end of the pull rod passes through the sliding chute and is arranged outside the drum body. The mixing drum is novel and unique in structure and safe in use.