The invention relates to the field of sugarcane harvesting, in particular to a sugarcane undercutting device. The sugarcane undercutting device comprises a hydraulic motor, a box and a cutter shaft transmission shaft; the sugarcane undercutting device further comprises a cutter shaft connecting plate welding assembly and a grass blocking mechanism, the grass blocking mechanism is arranged on the outer side of the cutter shaft connecting plate welding assembly, and the upper portion of the grass blocking mechanism adopts a four rib loop lap structure; by means of the structure, the weight of the whole device is greatly lowered, in the movement process, the cutter shaft transmission shaft can be protected against winding of weeds and dry vines, the cutter shaft transmission shaft rotationalfailure risk is greatly lowered, four streamline scrapers are uniformly distributed on the grass blocking mechanism, tooth plate mounting holes are formed in the scrapers, tooth plate installation isfacilitated, and then sugarcane cut down feeding is achieved; when the cutter shaft transmission shaft rotates, the grass blocking mechanism is driven to rotate, the weeds are effectively stopped fromwinding around the cutter shaft transmission shaft, and the reliability of the cutter shaft transmission shaft is guaranteed.