The invention relates to a cotton knitwear activated dye imitation old dyeing method which is characterized by including steps: 1) pretreatment of a gray fabric; 2) removal of wool; 3) dyeing of activated dyes; 4) treatment by the aid of sodium bicarbonate; 5) water washing after comparison between a sample and a dyed fabric; 6) treatment by the aid of glacial acetic acid; 7) taking out of the dyed fabric from a cylinder and dewatering of the dyed fabric; 8) dying; 9) product sizing at the temperature of 130 DEG C. The method changes a present situation that a conventional activated dye cannot make imitation old color, wool on the fabric surfaces is removed after the gray fabric is pretreated, the activated dyes are treated by the aid of the sodium bicarbonate after dyeing and washing with a soap, so that activated colors are in weakly-alkaline solution, the colors change dark under influential effects of temperature and time, the dyed fabric is circularly dried in a loop drying machine, so that the dyed fabric can mutually rub, fabric surfaces have whiting fuzzy effects, and finished products are sized by adding steam in a dry manner, and cotton knitted gray fabrics have imitated old feel.