Method for extracting plant dye by transforming Rubia cordifolia by microorganisms

The invention discloses a method for extracting plant dye by transforming Rubia cordifolia by microorganisms. The method includes the following steps that step 1, the mature madder roots of Rubia cordifolia are taken, and then pulverized into madder root granules with 40-60 mesh; step 2, the madder root granules are cultured by microorganisms, so that the madder root granules are cultivated to produce rubican and a small amount of free alizarin to obtain the madder root attached with rubican and a small amount of free alizarin; step 3, the madder roots attached with rubican and a small amountof free alizarin are fermented by biological enzymes so as to cut off glycosidic bonds of the rubican, so that the rubican is converted into the alizarin, and the madder roots attached with the alizarin are obtained; and step four, extraction and concentration of the alizarin are carried out on the madder roots attached with the alizarin to obtain a colorless extracting solution, the colorless extracting solution is conce