The utility model discloses a seat shaft structure of a pumping unit brake middle seat. Two ends of the seat shaft are provided with keyways. The position where a seat shaft of the middle seat is filled with a butter bore is provided with a ring recess. The surface of the seat shat is provided with at least one strip of groove along an axial direction, and the ring recess is communicated with the grooves. A butter filling channel is formed by the ring recess and the grooves. A part of butter can be effectively stored by the ring recess and the grooves. When the butter is filled, resistance can be easily reduced and dead oil can be effectively eliminated, therefore the brake middle seat is prevented from being rusted and stuck.