Cotton defoliating agent

The invention relates to a cotton defoliating agent. The cotton defoliating agent comprises, by weight, 8 to 20 parts of thidiazuron and 80 to 92 parts of magnesium chlorate. The cotton defoliating agent is used when cotton bolls crack at about 70%, defoliation is started after 3 days, the cotton leaf defoliating rate reaches over 85% after 6 days, the cotton leaf defoliating rate reaches a peak after 10 days, and the cotton leaf defoliating rate reaches over 95%. The cotton defoliating agent is free of harm to a plant and boll opening is normal.