Abstract: In order to rationalize the design of the straw-mulching cutter shaft of the whole straw-returning machine (Model: 1ZT-210), fuzzy factors in the design were fully considered, and the wall thickness of the cutter shaft was obtained by fuzzy comprehensive evaluation; and the reliability sensitivity matrix of the random variables effecting the cutter shaft’s function was established. The design information of the reliability sensitivity of the cutter shaft was obtained by the computer programming. Based on the results of sensitivity analysis, extract reliability design parameters were selected, then the cross-sectional area of the cutter shaft was reduced by 39.17% compared with conventional design. A numerical example demonstrates that the result of presented method is accord with the qualitative analysis; the proposed method is efficient and accurate. The results of the field experiment manifest that the reliability sensitivity design makes the life of cutter shaft increase 560 hours, and the pass rate of weed buried raises to 95%.