Abstract: In the design process of four-twine big rectangular bale pickup baler，taking piston as the object of study which was key working parts of compression mechanism，3 dimensional model of piston parts and assembly was established by SolidWorks software.Dynamic simulation on piston was made by ADAMS software.Simulation time was 5 seconds and piston compression frequency was 42 rmin.Simulation results showed that piston stroke was 710 mm，maximum value of piston compression pushing speed was 0.85 ms，maximum value of return speed was 0.91 ms，acceleration range of piston was from －1.58 ms2 to 2.21 ms2，maximum value of the force in X direction of hinge of piston and connecting rod was 2 788 kN，torque range of crank uniform speed rotation was from －264.4 N·m to 174.4 N·m.Simulation results provided theoretical basis for optimal design of compression mechanism of four-twine big rectangular bale pickup baler.It shortened design cycle and improved design efficiency.