Accessed to literatures at home and abroad, we introduced the process of indigo naturalis transforming from dyestuff industry into pharmaceutical industry. It is affirmed that the precursors of indigo and indirubin are isatan A, isatan B, isatan C and indicant. Meanwhile, author clarified the mechanism of transformation for these precursors transforming into indigo and indirubin. And we summarized methods of determination for these precursors. In summary, these references provide us accordance of study on processing principle of Indigo naturalis, and lay the foundation for technics of making indigo and indirubin entering into modern industry.