The invention relates to the technical field of plant extract preparations, and concretely relates to a plant extract nanoparticle, and a preparation method and an application thereof. The plant extract nanoparticle is a Krameria triandra extract nanoparticle or a totarol nanoparticle concretely, and is prepared from a plant extract, a stabilizer and a solvent. The plant extract nanoparticle solves the problem of poor solubility of the Krameria triandra extract and totarol and the problem of clinical and practical application difficulties caused by the poor solubility, has the advantages of good particle size uniformity, narrow particle size distribution range, non-agglomeration, high stability, freeness of organic solvents, small dosage of a surfactant, and high safety, and is suitable for being used alone or compounded with other active ingredients.