The invention provides a treatment agent and a treatment method of reducing a nicotine content of an upper tobacco leaf of a cured tobacco and belongs to the field of plant growth regulation. The treatment agent and the treatment method solves the problem of influence on the quality of the tobacco leaf due to quick increase of the nicotine content of the upper tobacco leaf after topping. The treatment agent is a mixture of salicylhydroxamic acid and glucose. The invention further provides the treatment method of reducing the nicotine content of the upper tobacco leaf after the topping. The method comprises the steps of dissolving salicylhydroxamic acid and glucose in agar gel to prepare an agar gel block, fixedly placing the agar gel block at a cutting surface of a topping stem of a tobacco plant, or spraying a mixed solution of salicylhydroxamic acid and glucose onto the upper tobacco leaf after the topping of the tobacco plant and harvesting of a lower tobacco leaf. The treatment agent of reducing the nicotine content of the upper tobacco leaf of the cured tobacco is simple in formula, significant in effect, and flexible in use method, can significantly reduce the nicotine content of the upper tobacco leaf after the topping and improves the quality of the tobacco leaf.