A shift lever interlock part-bundle mechanism for hay baler was designed and the movement relationship between lever groups and compression mechanism was established. In order to reduce falling-board time and ensure non-interference of movement between separation board and compressive piston an eject utensil style inserter was designed its movement was regulated. By using Matlab/Simulink imitation, the main factors influencing falling-board time was obtained, based on which the structure design was improved and the reliability was enhanced. The experimental results demonstrated that the mechanism worked well.