The invention provides a medicament for treating idiopathic edema and a preparation method thereof. The medicine contains pure natural plant drugs of angelica, radix bupleuri, atractylis ovata, Alisma orientale, radix paeoniae alba, tuckahoe, grifola, rhizoma cyperi, motherwort, trumpetweed, cassia twig, and the like. In the medicament, the angelica breaks extravesated blood, nourishes new blood and relaxes vain to eliminate dampness; radix bupleuri, tuckahoe and Alisma orientale soothes liver and strengthens spleen for inducing diuresis to alleviate edema; the atractylis ovata, grifola, and the like, invigorates qi and strengthens the spleen, clears damp and promotes diuresis and blood circulation, and recuperates the body function; the plant drugs are combined to free liver qi and strengthen the spleen, smooth collaterals, avoid moisten, and eliminate edema, so as to ensure the effect of sooth liver and strengthen spleen, smooth collaterals and induce diuresis. Research shows that the medicament has the pharmacological actions of inducing diuresis, reducing capillary permeability, improving microcirculatory disturbance and increasing immunologic function, and the like, treats idiopathic edema etiologically and semeiologically, and has the advantages and characteristics of multiple levels and treatment on principal and subordinate of traditional Chinese medicine.