The invention relates to a preparation method of insecticide for vegetable insect pests. The preparation method comprises the following steps: A. preparing glycyrrhizic acid, tea polyphenol, ginger juice, flos rhododendri mollis, ammonia water, Tripterygium wilfordii, fulvic acid and modified starch; B. adding water into fulvic acid to prepare a fulvic acid solution with the percent concentration of 10%; and adding modified starch into the fulvic acid solution and stirring uniformly for standby use; C. extracting leaching liquid of Tripterygium wilfordii and flos rhododendri mollis by using a water extraction method, mixing glycyrrhizic acid, tea polyphenol, ginger juice and water leaching liquid and setting aside for use; and D. mixing a product obtained in the step B and a product obtained in the step C uniformly, and adding ammonia water until the ammonia water is dissolved sufficiently, thus preparing the insecticide for vegetable insect pests. The insecticide has reasonable proportion and can be used conveniently, botanical materials are used as main materials for preparing the insecticide for vegetable insect pests, and the pollution degree is low.