Abstract: Abstract: At present, only movement state of single threshing rotor is considered for threshing system power model in simulation design of speed control system for combine harvester, and movement states of other work parts and impurity quantity in threshed materials are not considered, so it is necessary to make further study on theoretical model of threshing system in order to improve simulation design of speed control system and subsequent optimization of control algorithm. In this paper, 3 fundamental hypotheses were made as follows: 1) Crop was fed continuously and evenly into the threshing system and crop moisture was not taken into account; 2) Crop flowing was constant and continuous in threshing space, and there was no relative sliding between crop layers; 3) The threshed materials were separated from concave, the speed of which was equal to the peripheral speed of threshing rotors. And taking the XG610 combine harvester as example, the kinetic model for threshing system was established based on kinetic analysis of work parts, of which equivalent device 1 was mainly composed of reel, cutting table auger-type conveyer and conveyer trough, and equivalent device 2 was mainly composed of cleaning mechanism and grain auger-type conveyer, and intermediate shaft. Then the simulation model of speed control system was constructed based on the combination of the fuzzy logic controller and the kinetic model of threshing system. At the same time the simulation subsystem of feedback element was also built based on the kinetic model formula. In the design process of the fuzzy logic controller, the variables were input, including threshing rotor rotation speed deviation and deviation variation rate, and the output variable was the rotation angle of stepping motor by using fuzzy inference according to the corresponding input variables. The types of their membership functions were all triangular, and fuzzy inference system had 49 fuzzy rules. The simulation results showed that in beginning stage the threshing system was doing self-adjustment, then the threshing rotor's rotation speed dropped a little and kept stable at about 825 r/min, and the change of forward speed had the delay of 0.7 s compared to that of the threshing rotor rotation speed and was stable at about 2.0 m/s. And at the 25th second the cropping intensity had step change that it increased from initial value 0.95 to 1.09 kg/m2, which made the feeding quantity increase by about 15% compared to initial value and the engine would work in full load state, and the speed control simulation system made effective adjustment in about 5 s. It spent 1.5 s for crop flow from cutting table auger-type conveyer to conveyer trough and till into the threshing rotor space, and at the 26.5th second, the threshing rotor rotation speed began to fall again to about 780 r/min and was finally stable at about 803 r/min, and the forward speed, which had the delay of 0.7 s compared to the threshing rotor rotation speed, began to fall to about 1.90 m/s and was finally stable at about 1.99 m/s. The above changes of threshing rotor rotation speed and forward speed prevent effectively the occurrence of overload and jam of threshing system in working process, which shows that the kinetic model of threshing system is reasonable for speed control of the combine harvester. Experimental data also prove that the speed control system is feasible and the kinetic model is reasonable. Additionally, the threshing system kinetic model established can comprehensively reflect the working characteristics of XG610 combined harvester threshing system, and give a good reference model of threshing system for other types of combine harvester.