The invention relates to the field of preparation of rare earth, in particular to a herbaceous plant rare-earth precipitating agent and a preparation method thereof. The herbaceous plant rare-earth precipitating agent is prepared from, by weight, 30-50 parts of straw, 25-65 parts of fibrous stalks, 0.5-5 parts of herb for clearing away heat, 0.5-5 parts of Vitex.negundo L.var. cannabifolia (sieb. et Zucc.) Hand.-Mazz. herb and 0.08-5 parts of fungi, and the raw materials are weighed, smashed, fermented and soaked so that the herbaceous plant rare-earth precipitating agent can be prepared. By means of the herbaceous plant precipitating agent, the mother liquid precipitating effect can reach 99.6% or above after mineral leaching is performed on rare earth minerals in different types, the concentration of ammonia and nitrogen in supernatant after precipitation is 0.94 mg / L, secondary pollution of ammonia and nitrogen is avoided, herb precipitate contained in the supernatant can be directly used circularly, and the herbaceous plant rare-earth precipitating agent is a natural and environment-friendly precipitating agent.