Acid dye refers to the dyestuff containing a sulfonic acid group, a carboxyl, a hydroxy or other soluble group in the molecular structure and used in acid or neutral dye bath. According to the chemical structure it can be divided into azo type, triphenylmethane type, anthraquinone type, anthracene oxygen type and nitroso type pyrazolone type. Acid dyestuff is widely used, mainly as direct dyestuff for protein fibers (such as wool, silk) and polyamide fibers, protein. And the dyeing needs to be conducted in an acidic solution. It’s mainly used for nylon, wool, leather, silk and others. Direct dyes or basic dyes are applied in paper dyeing far more often than acid dyestuff.