The invention relates to a medicament for treating kidney diseases and a preparation method thereof, and belongs to the field of Chinese medicaments. The medicament is characterized by being prepared from the following raw materials serving as active ingredients in part by weight: 30 to 50 parts of astragalus, 30 to 50 parts of motherwort, 10 to 30 parts of lalang grass rhizome, 10 to 30 parts of polygonum aviculare, 10 to 30 parts of dianthus superbus, 10 to 30 parts of field thistle, 5 to 15 parts of pyrrosia leaf, 5 to 15 parts of garden burnet charcoal, 10 to 30 parts of dock root, and 2 to 4 parts of liquorice. The preparation method comprises the following steps of: (1) performing water extraction and alcohol precipitation on the astragalus, and filtering the extract by using a macroporous resin column to obtain an astragalus extract; (2) adding water or alcohol-water solvent into the motherwort and extracting, and filtering the extract by using an ion exchange resin column to obtain a motherwort extract; (3) adding water or alcohol-water solvent into the dock root and extracting, filtering the extract by using a macroporous adsorption resin or polyamide resin column to obtain a dock root extract; (4) performing water extraction and alcohol precipitation on the field thistle, the lalang grass rhizome, the polygonum aviculare, the dianthus superbus, the pyrrosia leaf, the garden burnet charcoal and the liquorice, to obtain seven extracts; and (5) mixing the four extracts obtained by the steps uniformly to obtain medicinal active substances. The medicinal active substances are used as medicinal effective ingredients and prepared into a preparation by using the conventional technology of pharmaceutics.