The invention relates to a plant pesticide and its preparation method. The pesticide is prepared by extraction processing of herb of Suaeda salsa. Main active components of the plant pesticide comprise: by weight, 15-20% of tamarixetin-3-O-rutinoside, 5-10% of genkwanin-4'-O-beta-D-rutinoside, 5-10% of syringaresinol-4-O-beta-D-glucoside, 5-60% of other phenolic acids, water and a surfactant. The above components are made into an emulsion in water, and the emulsion in water is diluted to be used for killing insects. The preparation method provided by the invention has advantages as follows: the raw materials are easily available; the technology is simple; and costs are low. The plant pesticide prepared by the preparation method has a very good effect of controlling aphid, armyworm, spider mite, rice stem borer and a plurality of noctuidae insects, has rapid speed of killing insects, is safe to crops, causes no pollution to the environment, and is a biopesticide preparation suitable for nuisanceless agricultural product production requirements of our country.