Abstract: In order to rationalize the design of the straw-mulching cutter shaft of the whole straw-returning-back-to-field machine (Model: 1ZT-210), fuzzy factors in the design were fully considered and fuzzy optimization design method together with reliability theory was applied to develop the model for fuzzy reliability optimization design with the minimum weight and maximum reliability of the straw-mulching cutter shaft as the objective function. The fuzzy optimization design result of the cutter shaft was obtained by compiling MATLAB procedure. Compared with traditional design methods, the cross-sectional area of the straw-mulching cutter shaft was reduced by 33.3%. The life and the reliability of the cutter shaft were raised by 38.5% and 0.8% respectively, and the wear quantity of the cutter was decreased by 40%.