The invention relates to the technical field of dyeing additives and especially provides a flatting agent for dyeing chinlon. The flatting agent is composed of lauryl sodium sulfate, tetrachloroethylene, glacial acetic acid, wax emulsion, methylbenzene, butanone, silicon dioxide, organic bismuth and water according to the following ratio, by mass, 2-4 parts of the lauryl sodium sulfate, 15-18 parts of the tetrachloroethylene, 15-20 parts of the glacial acetic acid, 5-13 parts of the wax emulsion, 1-3 parts of the methylbenzene, 6-10 parts of the butanone, 3-9 parts of the silicon dioxide, 2-5 parts of the organic bismuth, and 40-50 parts of water. The flatting agent can eliminate an aurora-like phenomenon on a chinlon fabric, thereby improving performance and quality of the chinlon fabric. The flatting agent is low in usage cost.