The invention discloses a natural plant defoliating agent, comprising the following components in percentage by mass: 0.5%-20% of abscisic acid, 0-20% of thidiazuron, 3%-7% of a wetting dispersant, 0.8%-5% of a thickening agent, 0.2%-0.4% of a defoaming agent, 0.2%-0.5% of a stabilizer, 0.3%-0.8% of an antifreezing agent and the balance of water. The invention also provides a preparation method of the natural plant defoliating agent. An abscisic acid existing in a natural plant is adopted as a main component; a herbicide component is not added; ethephon and an adjuvant are not added in use; and the content of an endogenous abscisic acid of the plant is induced to lastingly increase and keep in balance with hormone change by applying the abscisic acid, so that plant defoliation is achieved; the usage amount of a chemical defoliating agent and a chemical pesticide ethephon is reduced; the soil environmental pollution and residues caused by use of large quantity of the chemical pesticide are relieved; the cotton fiber quality is improved; and the agriculture production cost is reduced.