The invention relates to the field of natural dyes, and specifically relates to a natural plant dye, a preparation method and application thereof. According to the invention, the natural plant dye is obtained by extracting vicious weeds serving as raw materials, wherein the vicious weeds are one or more in a group including chromolaena odorata, mikania cordata, water hyacinth, alternanthera philoxeroides, alternanthera pungens, spartina anglica s, ragweed, lantana camara, solanum aculeatissimum and lolium temulentum. The extraction method comprises the steps of grinding and extracting through water. The invention provides a novel scheme for utilizing exotic vicious weeds; the preparation of the natural plant dye by taking exotic invasive weeds as raw materials can protect environment and bring conomic benefits; residue produced during dye extraction can be burnt and combustion can provide a heat source for a dyeing process; and obtained ash can be used as mordanting assistants for dyeing with no storage or treatment so as to really achieve environmental protection.