Plant hair dye and method for dyeing hair by the plant hair dye

The invention discloses a plant hair dye and a method for dyeing hair by the plant hair dye. The plant hair dye contains 1, a composition containing a water-soluble ferric salt and an excipient, wherein the weight percentage of the water-soluble ferric salt is in a range of 1 to 10%, and 2, a composition containing tannic acid or hematorylin and an excipient, wherein the weight percentage of tannic acid or hematorylin is in a range of 1 to 10%. The method for dyeing hair by the plant hair dye comprises the following steps of smearing the composition containing the water-soluble ferric salt and the excipient on semi-wet or dry hair, washing off the composition containing the water-soluble ferric salt and the excipient, smearing the composition containing tannic acid or hematorylin and the excipient on the hair, and washing off the composition containing tannic acid or hematorylin and the excipient. The plant hair dye also contains a composition containing an excipient, and D,L-cysteine and/or its salt, or a cysteine derivative, or thioglycolic acid and/or its salt or ester. The plant hair dye has the advantages of no irritation, no allergenicity, no mutagenicity, no stink, no smell, no damage on hair, excellent dyeability and color tone, and long color retention time after dyeing.