The paper designed a straw crusher device based on genetic optimization design method. Firstly, straw crusher device math model was build. Secondly, taking the cutter shaft as optimized object, the cutter shaft in the working device axis diameter, internal diameter and ratio were set as variables, and taking the lightest quality of cutter shaft as the objective function, to build the mathematical model of straw crusher. Then combined with the GA optimum design method, straw crusher optimization design model and simulation model were optimized. Lastly, the structural parameters of straw crusher and satisfactory results were get. The article improves the efficiency of design and the quality of product; it also provides a new kind of design method for agricultural machinery.