A very important reason why salt has a dyeing effect on direct dyes is to reduce the charge effect during fiber dyeing. Cellulose fiber has a surface potential in a neutral or weakly alkaline dye bath, and generates a charge repulsion with the direct dye anion. After the electrolyte is added, a large amount of positively charged sodium ions are generated in the dye solution, and the charge anion is kept neutral. The obstacles in the dyeing process are eliminated, the dye molecules are in close contact with the fiber surface, and the concentration difference between the dye liquor and the fiber interface is reduced, thereby increasing the dye uptake percentage. However, the dyeing process of direct dyes can be roughly divided into three stages: adsorption, diffusion and fixation. In the adsorption stage, the dye should not be colored too fast. If the electrolyte is added during the adsorption stage, the dye may be aggregated to cause uneven dyeing, and the electrolyte may be added in the middle to avoid excessive coloring and double dyeing. percentage.