The damage of sugarcane stubble affects the quality of its biennial root. Based on establishing the mechanics model of sugarcane stalk in the cutting, mechanics analysis on stubble damage was done. It was found that: the failure form of the stubble will be splitting and breaking when the stalk is cut by single blade; And it will be axial direction crack by torsional shearing stress, splitting and breaking by bending stress when the stalk is cut by two or more blades one by one. Formulas of shearing stress and bending stress were derived, and the failure criterion was given in the paper.