The invention relates to a preparation method of a slow-release pesticide. The method comprises the steps of: adsorbing a pesticidal active compound into a porous material; using a pore channel sealing material to seal the pore channels of a porous carrier; conducting further processing so as to obtain a suspending agent, powder, a granule, a tablet and other products of different preparation forms. The pesticidal active compound can be a pesticide, a bactericide, a herbicide or a plant-growth regulator. The preparation method of a slow-release pesticide in the invention can be used to prepare slow-release powder, slow-release granules, slow-release tablets, slow-release suspending agents and other products of different preparation forms so as to meet different demands of agricultural production, and can control the release of pesticidal active components into a surrounding environment at a predetermined speed or a low speed within a predetermined time. One-time application of a slow release pesticide prepared in the invention can control diseases, pests and weeds of a whole growth period (or a certain period). Thus, the slow-release performance of pesticides can be substantially enhanced, the release time can be prolonged, and long-time stable release of slow release pesticides can be maintained.