The invention discloses a threshing mechanism of a self-propelled flax threshing spreading machine, including a rack, a pair of smooth rollers for pressing flax arranged up and down and a pair of threshing awl rollers arranged up and down, wherein, the smooth rollers for pressing flax are driven by a pair of hydraulic motors, the rotation of the threshing awl rollers at same speed and in different directions is realized by a universal joint through two output shafts of a gear box which is driven by another hydraulic motor, and a plurality of stirrups are alternately arranged on the threshing awl rollers along the axial direction. The spreading machine picks up the retted flax in the field, transports the flax to a turning belt behind the machine by a delivery mechanism and puts the flax in strips; during the transportation process, a pair of smooth rollers for pressing flax grind the flax peaches, a plurality of smooth stirrups alternately arranged on the threshing awl rollers carry out hackling and threshing to the flax peaches, and the flax threshing in the flax turning process is realized. The threshing mechanism has simple structure, steady work, high threshing efficiency and small damage to the straws, and reduces the cost of activity and the labor intensity.