The invention relates to a synergism composition of a weed killer for a sugarcane field. Oxyfluorfen, halosulfuron-methyl and acetochlor are used as conventional weed killers which are used in a single manner, but can not effectively act and kill weeds such as cyperus rotundus. The synergism composition of the weed killer for the sugarcane field comprises an effective ingredient and auxiliary materials, wherein the effective ingredient is prepared from the following components by weight percent: 1.5-45 percent of oxyfluorfen, 1.0-20 percent of halosulfuron-methyl and 6-60 percent of acetochlor. According to the synergism composition, the oxyfluorfen, the halosulfuron-methyl and the acetochlor are prepared into a preparation according to a reasonable proportion, and the preparation has a remarkable synergism to the cyperus rotundus, and is capable of effectively killing the cyperus rotundus with the control effect of 95-100 percent. In addition, the synergism composition kills worst weeds such as oxalis corymbosa, rottboellia exaltata and echinochloa colona and other common weeds in the sugarcane field, and can achieve the weeding effect through once application in the sugarcane field and fill the blank of lack of a weed killer for effectively killing the cyperus rotundus in the sugarcane field at present.